Sometimes in science fiction, you'll find aliens whose bodies replace carbon with silicon. In hard science fiction, the plausibility of this trope rests on carbon and silicon being in the same family of elements: the ability to form four bonds opens the door to a vast array of potential complex nanostructures.
Silicon biochemistry would be wildly different though: you can't replace carbon with silicon in known biomolecules and get anything functional. Even the small "hydrosilicon" molecules are much more reactive than their carbon counterparts. Plus, carbon is actually not so common on the earth, with the SECOND most abundant element being...wait for it...siliconnote silicon dioxide is the most abundant compound on earth but as the name suggests, oxygen is at least twice as abundant as silicon . If that kind of life were able to form, it's reasonable to assume it would have done so here (but it, you know, didn't).
It's somewhat more chemically plausible to base life on silicates (silicon oxides), which have a huge variety in nature. These species would either have to exist at molten-glass temperatures, or be very slow (literally geological). Silicate based lifeforms would definitely be more Starfish Aliens than Rubber-Forehead Aliens.
Most of the examples below are not hard science fiction. Authors have a lot of fun with the possibilities. Sometimes other elements are used, which don't have to be in Group 4 of the periodic table.
See also: Living Statue, Golem, Rock Monster and Body to Jewel. Nothing to do with silicon chip based lifeforms or Pamela Anderson.
Suzumiya Haruhi: Silicon-based Data lifeforms were responsible for the mysterious happenings in one of the two stories of the eighth volume. But they live on without their bodies. How? That's classified, apparently.
Blame! features a race called the "Silicon Creatures" or "Silicon Lives". Like the rest of the cast, they're actually just advanced cyborgs who "evolved" from humans, but they're so heavily altered that it's hard to say if the fleshy-looking parts of them are really flesh or not anymore.
In Project ARMS, the alien life form Azreal is silicon-based. The discovery of it is what gets the Egrigori to kick-start their various ARMS experiments. It also saves the day at the end, when it's revealed that Azreal is nearly immortal because of its silicon makeup, which leaves it very lonely and more willing to sympathize with the protagonists than Keith White (who wants to kill everyone on Earth and essentially condemn Azreal to an eternity alone).
In another DC Universe story by Alan Moore collected in the same book, there is a rare example of a more realistic portrayal of silicon lives as they would exist at Earth-like temperatures. When their planet is invaded by the power-hungry Spider Guild, the silicon beings, resembling giant stone statues of men, don't even notice because they move at a geologic pace.
Stonians, from Astonishing Tales #22 and Strange Tales #74, are silicon-based lifeforms.
Superboy (Kal-El) had to save a race of silicon-based aliens called the Vulxans in The New Adventures of Superboy #7 (1980).
In "Under the Volcano", a comic story published in Doctor Who Adventures Magazine, the Doctor and Rose run into a silicon-based species called the Chalderans.
Stargate Atlantis: In the Dark Frontier fanfiction, the Shade are a strange hybrid of carbon- and silicon-based life: they are mostly carbon-based but use a powerful acid to digest any mineral in order to build and repair a silicon-based exoskeleton. Their unusual composition also gives them the ability to use ammonia and sulfuric acid for what we use water. Oh, and they are cold-blooded, capable of draining anything from alkaline batteries to energy shields in order to stay alive for months without feeding in any environment. The biggest ones (few thousand miles across) strip mine entire planets for naquadah and trinium, also draining the planetary core for heat; when they move on, the planet is essentially a dead husk locked in an eternal ice age while the metals are used to build planetoids which serve as hatcheries. Despite being living creatures, they are apparently capable of entering hyperspace.
Xenomorphs from the Alien franchise are apparently silicon based.
The giant space slug in Star Wars V , which is officially called an exogorth, is silicon-based, as are the winged creatures inside it, which are called mynocks. This detail is not mentioned in the film and comes from the Expanded Universe.
Sentenced to Prism by Alan Dean Foster. Most life on Prism is silicon based. Foster has a lot of fun with the possibilities: natural Frickin' Laser Beams, rich colouration, lots of armour, a casual approach to being on land or underwater (the inhabitants consider water 'thick air') and so forth.
Stanley G. Weinbaum's short story "A Martian Odyssey" had a silicon-based life form that consumed silicon and excreted silicon dioxide bricks.
Discworld: Played for laughs, but Trolls qualify. It is specifically noted that, as silicon-based organisms, they function like supercomputers, with processing speed (and thus intelligence) being inversely proportional to temperature. One of them uses Hulk Speak at room temperatures, but when locked in a freezer for a while, almost came up with a unified field theory. They also count in either base four or binary because of that.
I thought it was base many.
The third Young Wizards book has a planet of silicon-based life forms, the fact of which ends up being extremely important. As it turns out, they can function like extremely complex computers.
Later books mention in passing that to some silicon-based species, chocolate is an aphrodisiac.
The Dancing Meteorite by Anne Mason includes a flashback to humans' first encounter with silicon-based life, which the human explorers think is just part of the landscape until it rises up and attacks them.
Ullerans in H. Beam Piper's Uller Uprising. The concept is made more believable by using silicones (Si-O-Si-O-Si chains), some of which are pliable at what we consider room temperature. Piper didn't come up with idea - he was presented with an introductory essay by Dr. John D. Clark describing life on both Uller and Niflheim (see entry below under "Other Elements".
Dark is the Sun by Philip Jose Farmer: Phremompit is a silicon based lifeform native to an asteroid, coming to Earth in a meteor shower. He eats radioactive rocks and moves on natural treads. Unfortunately, he drills through many people before learning his morse-code communication lazer is turned up a bit too strong for the mushy-bodied earthlings.
In Ben Bova's Venus, it's discovered that such a lifeform inhabits the titular planet. It has a massive underground body, and tentacles a few kilometers long which it uses to search the planets surface, for food or whatnot.
The vampiric beings from The Stress of Her Regard appear to be this trope, although it's slightly too early in history for the characters to describe them in terms of modern chemistry.
Although no actual life form appears, Sheldon of The Big Bang Theory builds a model of a hypothetical silicon-based DNA analogue.
From the same series, a sapient race called the Strata.
In The Laundry Series, there are creatures living in the Earth's mantle— codeword DEEP SEVEN, or "Cthonians". True to the more realistic version of this trope, they freeze to death at any temperature where rock is a solid. True to the general tone of the Laundry Files, they're massively more technologically advanced and numerous than us, have insanely powerful war drones that are fully capable of coming up to the surface to squash us, and may realize any day now that they have very good reason to want us squashed.note They even scare the crap out of BLUE HADES, who also meet the description in the previous sentence exactly.
Live Action TV
The Tholians of Star Trek, as seen in the page picture (taken from Star Trek: Enterprise). The original Trek just showed the head. The Horta are also this trope.
The extinct Martian insects that are sometimes displayed on computer screens were, according to certain reference books, also silicon-based lifeforms.
The "Crystalline Entity" that wiped out all life on the planet Data was found on.
The X-Files: The creature from "Firewalker" was a silicon based fungus-like creature that exploded from people's throats to spread its spore.
Doctor Who has several examples, including the Kastrians from "The Hand of Fear" and the Ogri from "The Stones of Blood".
Disneyland: "Mars And Beyond" briefly mentions silicon based life in a segment about what life on Mars would look like. It suggests that it would take the form of crystalline structures that rise during the day, then shatter into oblivion in the cold Martian night.
Carl Sagan explored this question in this video: "I wonder if we'll ever find a specimen of life based not on organic molecules, but on something else — something more exotic." He goes on to describe himself as a "carbon chauvinist."
Classic Journal of the Travellers' Aid Society #15 article "The Bestiary". Doyle's eel is a silicon-based life form that eats metal. If it infiltrates a starship it will try to eat metals, silicon and some plastics, which can cause serious damage. It will also try to lay eggs and hatch out more of them which will cause even more damage.
Marc Miller's Traveller supplement Milieu 0 Campaign, chapter "The First Wave". In one of the Survey adventures the PCs can encounter large rock-like creatures that are silicon life-forms. They are serpent-like and can project lava jets as a weapon.
Gunman Chronicles uses it as a plot point, where Silicon lifeforms can't digest Carbon lifeforms.
The Taalo in the games' backstory as well, though they were living rocks rather than specifically crystals. Their unique physiology is what enabled them to make friends with the Ur-Quan (who, being predators, usually feel an overwhelming instinct to eat other creatures they come into contact with... but rocks don't provoke that instinct).
Master of Orion: The Silicoids. The wildly divergent biochemistry of silicon-based organisms is reflected in the gameplay. Silicoids do not need to farm to produce food, thus freeing up their population to conduct research or construction. In addition, they are immune to the effects of pollution and can live on even the harshest planets without a problem. On the down side, their slower metabolism means slower population growth, and their vastly different psychology makes it nearly impossible to deal with other races diplomatically.
Also, the Space Crystal monster, which flies around and exterminates the population of colonized planets.
Metroid Prime Hunters has this in Spire, whose race (the diamonts) are silicon based lifeforms. Super Metroid has Crocomire.
The Gastro, from The Maw, are silicon-based organisms.
Sword of the Stars has the Swarm, beelike things that spawn from asteroid hives and fire plasma, as well as sending out Silicoid Queens to build new hives. They are Demonic Spiders, especially in early game or with small groups, but reward you with a fair amount of resources if you clear a hive.
Technically, most Rock-type Pokémon count, as several of them are literally made of rocks. Same goes for many Ground-type Pokemon.
The Silacoids from X-COM: UFO Defense are simple silicon-based life forms that look like lumps of purple lava. They have rock-hard skin that is immune to fire damage and attack by biting. Because of their extremely high body temperature, they leave burn marks on the ground as they move, sometimes setting fire to nearby objects.
In XCOM: Enemy Unknown, researchers are forced to admit that they have no idea whether the Cyberdisks are living or purely mechanical, but conclude that if they are living, then they're probably silicon-based lifeforms. The Sectopods, on the other hand, are alive, and are silicon-based.
Muv-Luv: The BETA's creators are this, and believe that only Silicon Based Life is real life. To the degree that carbon based life isn't even considered alive. [[spoiler: In other words, the BETA don't even think they're fighting a war. From their point of view they're recycling resources.
In Duke Nukem Forever, on the alien ship there are breasts growing out of the wall. Duke will make a quip if you slap them, one of which is "Hmm, some strange silicon(e) based life form."
An enemy in Borderlands 2 are "crystalisks", which are tripodal organisms made of living rock and crystal — the base of each leg features a large patch of golden crystal, their only vulnerable spot, and they drop over a dozen valuable crystal shards upon death. One quest, leading into a long-abandoned mine called the Caustic Caverns, revolves around recovering an Apocalyptic Log that reveals that the crystalisks are actually highly intelligent and were friendly when humans first arrived. The security forces for the first mining expedition even made friends with them. When the profit-orientated mining officer demanded that the crystalisks be harvested, the security officers tried to revolt, but crumpled after their leader was killed. The crystalisks angrily rose up in self-defense and slaughtered the mining team, resulting in the closing of the mine, and they've been hostile towards humans ever since.
Schlock Mercenary: A halfway house between this and regular life: Sergeant Schlock, who is carbosilicate (i.e. based around both carbon and silicon). In a bit of realism, his biology and biochemistry is pretty clearly shown to be fundamentally different from traditional carbon-based life.
Vexxarr has two silicon-based species, both living in space. "Rock crabs" such as "Sid" (Silicon dioxide, get it?) are practically indestructible and feed off of radiation, while silicoid predators eat rock crabs and explode if fed cake. Both species are typically found roughly human-sized but grow to the size of asteroids.
Invasion America has a genetically-engineered species called manglers. A baffled human scientist describes the discovered bones of one as a "silicon analog" rather than carbon-based.
This trope is invoked in the episode of The Simpsons when Homer encounters an alien.
Homer: I'd be happy to answer your questions about the alien. Any questions at all. Dr. Hibert: Is the alien carbon based or silicon based? Homer: Uh ... the second one. Silliphone.
In one episode of the Flash Gordon animated series, Ming the Merciless created a silicon based monster and unleashed it on the heroes. The heroes weapons had no effect on the creature because, in their words, they were shooting sand.
Silicon is more abundant than carbon on the Earth's surface, and yet Earth life is almost exclusively carbon-based. The reason for this is because using silicon instead of carbon is only an advantage in hellishly hot environments (around 300 degrees Fahrenheit), where silicon compounds would remain stable while carbon compounds would break down. It's speculated that such silicon-based life would actually be silicone-based life, as silicones can bond with organic compounds just like the carbon backbones in our biology (silicones are silicon-oxygen chains used in synthetic rubber, engine lubricant, and breast implants). It would use a substance such as sulfuric acid as a solvent in the same way we use water (as it remains liquid under high temperature) and breathe pure fluorine (which has an attraction to silicon, and unlike oxygen, doesn't combine with it to form sand). To compensate for the vastly different methods of energy production and consumption, it would probably have physiology most closely comparable to fungus if not something stranger (the more popular crystalline formations would only be found in pure silicon-based life, not silicone-based life). Hilariously, they'd be prone to exploding on contact with an earth-like atmosphere.
Even weirder are the hypothetical sulfur and fluorosilicone based-life, who would live in even hotter environments, and at the hotter end, use molten rock as a solvent instead of water.
While life on Earth is carbon-based in its molecular structure, one origin-of-life theory posits that organic molecules could've initially begun copying themselves on the surfaces of wet clays. As clays do contain silicon, this would make "silicon-based life" true in a literal sense.
Some Real Life organisms use silicon in their skeletal structure (sponges) or as an abrasive to render themselves less palatable as a food source (grass).
Animorphs: While they don't discuss what element they're based on, the Venber in one story are "not carbon-based" as they melt above freezing temperatures. Probably ammonia. It's been speculated that ammonia-based life could exist on extremely cold planets.
Another story explores the possibility of hydrogen based creatures on the surface of the sun.
In one Isaac Asimov story, it's mentioned that some of the native bacteria on the Spacer planet Aurora have fluorocarbon rather than hydrocarbon chemistry.
H. Beam Piper's Uller Uprising briefly mentions life on the planet Niflheim. It's carbon-based, but uses fluorine in place of oxygen. Water is replaced by hydrofluoric acid and carbon dioxide by carbon tetrafluoride. Piper didn't come up with idea - he was presented with an introductory essay by Dr. John D. Clark describing life on both Niflheim and Uller (see entry above under "Silicon").
The Outsiders of Larry Niven's Known Space series are effectively bags of cryogenic superfluid helium. Their life functions and intelligence are entirely dictated by the interaction of currents inside their bodies.
Live Action TV
The X-Files episode featured ammonia-based alien brain worms that rode to Earth on an asteroid in prehistoric times and were hiding out at the North Pole. How they were able to take over the bodies of the human scientists that were trying to study them, despite the fact that not only is their biochemistry obviously incompatible, but being inside a human body for any length of time should have melted the little bastards is never adequately explained.
The Raxacoricofallapatorians (the Slitheens' species) are calcium-based.
Stargate SG-1 has sulfur-based aliens. The aliens themselves didn't show up as anything more than a picture, their automated probe was xenoforming an earth like planet into something their ecosystem could inhabit.
GURPS: Space has suggestions for hydrogen and sulfur based lifeforms as well as silicon based ones.
Star*Drive uses a page in its Alien Compendium sourcebook to explain six different classes of biochemistry, based on "liquid medium," "reagent for cellular respiration," and "compounds or elements that can create very complex organic molecules," which correlate well with the GURPS supplement described above. Furthermore, each type is only found on certain classes of planets. These classes are Class I: Terran (habitable), Class II: Minimal (minor life support required due to climatic extremes, atmospheric conditions, etc.), Class III: Extreme (major life support required due to intolerable climate or atmosphere), Class IV: Space (including asteroids, rings, etc.), and Class V: Jovian (extreme life support required).
The series of life are Series I: Water medium, Oxygen reagent, Carbon structure, Class 1/2 environment; Series II: Ammonia medium, Hydrogen reagent, Hydrocarbon structure, Class 3 environment; Series III: Water medium, Chlorine reagent, Carbon structure, Class 2/3 environment; Series IV: Sulfur dioxide medium, Sulfur trioxide reagent, Carbon structure, Class 3 environment; Series V: Sulfuric acid medium, Oxygen reagent, Silicone structure, Class 3 environment; Series VI: Sulfur medium, Sulfur dioxide reagent, Fluorosilicone structure, Class 3/5 environment; and Series VII: anything that doesn't fit into the biochemistries described above.
The volus of Mass Effect hail from a planet with an ammonia-based biosphere and a high gravitational field. This means that the volus can't survive in the oxygen/nitrogen atmosphere most other races, including humans hail from. Volus need to wear high-pressure suits at all times, which not only lets them breathe gasses that are more tolerable to their physiology, but also keeps them from exploding in a low-pressure atmosphere.