In the Marvel Universe, humans are the average between two secret sister races, the immortal Eternals and the monstrous Deviants. Despite their powers and technology, we came to dominate Earth simply because of our much higher birth rates, along with the fact that we've developed technology and powers of our own that allow us to keep up with them both, as well as the Deviants' war keeping their numbers down and the Eternals' lack of interest in anything that doesn't concern their city.
In the 1989 film Arena, there is an intergalactic fighting championship. the film states that there has been no human champion in over 50 years, as well as featuring many creatures nastier than humans, along with many who were wimpier.
The Lord of the Rings: Humans are average even in size. Their distinguishing trait as listed by Treebeard is that they are "Master of horses".
Diana Wynne Jones played with this trope in Power of Three, where the main protagonist race seem to be the human stand-ins in a world where there are also fairies and giants. They are 'normal', in-between the 'big people' and the 'little people'. Turns that the world is actually our world, or one close enough to it, and the 'giants' are actually humans, whereas the race we thought were humans are more akin to the small, shy and secretive elves of folklore.
In Alan Dean Foster's The Damned trilogy, this is subverted in the fact that humans are both average in ability— they can run, AND swim, AND climb, AND can adapt to climates wet and dry, hot and cold, etc.... yet this all forms a synergy with their psychology to make humans the deadliest damn warriors in the galaxy. (Of all the species in the universe, humans are the only ones who THRIVE in combat. To the other sentient races of the universe we are the equivalent of a Bengal tiger with tools.) To add a bit of background, most races in the galaxy are unable to even harm another sentient being. One species literally goes catatonic when viewing violence, and similar reactions of disgust are the norm. It's the area in which humans are not average that helps them here, as they are considered borderline insane for possessing individuals who find pleasure in war.
In Halo: Glasslands, Jul'Mdama remarks that humans don't need to be really, really good at one thing in particular in order to survive and achieve galactic domination. They just need to be average in a sizeable range of vocations.
The Mote in God's Eye: The Moties, who suffer from Crippling Overspecialization, see the humans this way: "They are amateurs at everything, second-best at everything they do." The fact that a single human being can be second-best at many roles, instead of being highly specialized, proves to be an advantage, of course.
Stargate SG-1's humans pretty much count. They use this to become a major power in the universe, more or less, after the Asgard begin to trust them.
In Farscape, despite Crichton's assertions to the contrary, what Humans lack in speed, strength and intelligence compared to most of the races in the universe, they make up in sheer tenacity.
In 1st and 2nd, Humans don't get any special bonuses or penalties, but can be of any class and have no limits on what level they can achieve in any class.
In 3rd Edition, humans get a bonus feat at first level, an extra skill point at every level (four at first level), and are easier to multi-class because whichever class they have the most levels in is treated as their racially favored class. In addition, the Chameleon class can only be taken by humans and doppelgangers (along with changelings, who are hybrids of the two).
In 4th Edition, humans get +2 to any one stat of your choice (other races get +2 to two fixed stats), +1 to non-armor defenses and an extra feat, trained skill and first level at-will power. That last one can be a major advantage, depending on class. In addition, the human-specific feats aren't specialized. Dwarves get feats that boost damage with hammers and axes, elves get benefits with swords and spears. Humans can take Action Surge, which gives +3 to hit to all attack rolls when they spend an action point (about once per two encounters), but this can apply to any attack rolls each time (bull rushes, weapons, spells...)
Humans are also "average" in that there is very little roleplaying material given for them. For example, dwarves, elves, and halflings all have their patron gods and typical alignments, but humans are just considered to be widely varied in their culture.
Warhammer: The Empire in a nutshell. Cavalry? Yeah, they're alright, but they pale in comparison to Bretonnian knights. Archers? Sure, just remember that Wood Elves are the kings of pointy stick death. Guns and steam-powered tanks? We have those too, but Dwarves do it better. Magic? Yep, but don't get into magic duels with High Elves. Swarms? We can just throw guys at you, but Skaven and Undead can throw even more. Melee? We're okay at that, but it's the Chaos Warriors who are the real face-wreckers of this game. The thing is, even though we're not especially great at any of these, we can do all of these to a competent degree. As a great general once said, "attack the enemy with the weapon he lacks", and the humans of the Empire are the undisputed kings of doing just that.
In the Warhammer spin-off Blood Bowl, the 'average' stat-line for a blood bowl player is movement 6, strength 3, agility 3 and armour 8. This, incidentally, (because it is) is the stat-line of the human lineman. Teams made up of humans are the empire, amazon and norse teams, where practically all the players are within one stat-point of the average (the empire teams have fast but fragile catchers and faster-than-average blitzers, while the amazons and norse take to the field in less armour. The norse can also hire werewolves).
There's also the Space Marines. They are the Jack of All Stats of all armies in the setting, although they are augmented so much they barely count as human anymore. If you count them as human, this trope is actually played straight.
In GURPS humans are the template that everything else is based on. By definition all other races (even ones functionally identical to humans) have some advantages and/or disadvantages relative to humans. A zero-point cost human in GURPS would have average-level basic stats and nothing else, not even skills. A Player Character, even one without special powers, costs much more.
From the behind-the-scenes point of view, the prevalence and variety of humans does a couple of good things for the game. First, it lets us put a human face on every color of Magic. That helps the look and feel of all the colors stay appealing to a wide variety of players. Our market research shows that we have a lot of human beings among our consumers, and having human beings in the art gives those players a familiar face that they can identify with. [...]
Second, humans play an important role as a point of comparison in every color. You get to see how tall or tough or magic-inclined goblins are compared to humans, for example, since you get to see them next to red-aligned humans that live in similar environments and have similar color values. You get to see what role griffins or pterons or leonin play in a given setting, because you get to see white-aligned humans riding them or hunting them or making alliances with them. We can afford to get more exotic with our nonhuman races, in part because there are plenty of examples of humans next to whom you can see similarities and differences—and we like that.
In Shadowrun, humans are the default race. All other "metatypes" cost character creation points. Humans receive no attribute bonuses or penalties, except they start with an extra point of Edge, which is basically luck.
Averted and/or Defied in Deep 7's games, like Arrowflight and the Red Dwarf RPG. Humans in Arrowflight tend to be more suceptable to getting Drunk on the Dark Side than most "Order" aligned races, but also tend to have greater tolerance to climate variations and poisons. Red Dwarf gives humans bonuses to endurance and fortitude because they're the species crazy enough to enjoy things like Yanni concerts and five-alarm curries.
Played with in World of Darkness. To make a supernatural character, you start with a basic human for a template, then add all the supernatural perks and additions. In actual Gameplay, though, even regular humans can be very dangerous to an unwary vampire or werewolf. The only real limitation that a human has is how easily they die compared to everything else. Other than that, a large enough and well prepared group of humans can take out a werewolf without breaking much of a sweat. Taken to its Logical Extreme with the Hunter class; completely regular humans trained and outfitted specifically for the destruction of supernatural monsters of all sorts.
Averted in Myriad Song, humans have better vision than other species and bonuses to Negotiation, Questioning, and Tactics with the Leadership and Low Profile gifts. To reflect how they were the favored servants of the setting's Abusive Precursors and not only widespread but the aristocracy of The Remnant.
Starflight: Humans are pretty much the most average species you can select for a crew member. For a starting ship, they can handle most roles competently (and are the best at Science), but once your ship has more capital, you're best getting a mix of species to have the most effective crew. In 2, humans are a liability at the end of the game, as bringing them into the main nebula will cause them to go insane and force the rest of the crew to quarantine them, and your ship will be lost if you have an all-human crew.
Averted in The Elder Scrolls—the Dunmer are the ones who are the Jack of All Stats. What's more, there's no one single human race to choose from—instead, there's several ethnicities that each have their own special perks.
Surprisingly not the case in Master of Magic. Humans (High Men) are balanced in their stats, but they trade off the ability for building some endgame buildings for a handful of elite units. On the other hand, orcs have no strikingly elite units but can build every town improvement in the game, and their combat stats are also average.
The third installment of Star Control, notably as explained by the human representative regarding the Earthling Cruiser.
SaGa games (particularly the early ones) use this as part of their Class and Level System. Humans can equip anything they want, but have meh stats and are totally at the mercy of the RNG for getting stronger. All other races get awesome abilities at the sacrifice of something else equally useful.
The Terrans from StarCraft are descendants of human colonists who originally came from Earth. By comparison to the other races in the setting, humans come off as rather weak: their technology is nowhere near as advanced as that of the Protoss and they lack the biological prowess of the Zerg. However, whatever technology they do have allows them to make cloaking devices and powered armour and employ a variety of powerful ranged weaponry. One biological advantage that Terrans do happen to have over the Zerg is that they're the only race—other than the Protoss—to have innate psionic potential, which is one of the main reasons why the Zerg seek to assimilate humanity in the first place.
In gameplay however, humans subvert this. While numerically, they are the average faction, with the Zerg being the Trope Namer for Zerg Rush and the Protoss being the Elite Army, Terrans as a whole are oriented towards Glass Cannons. All Terran units that can attack except for the SCV have ranged attacks and on average, terran units have less hit points than their alien counter parts.
Justified in Anarchy Online: all four of the playable genetic breeds are homnids, but the original is Solitus, the direct descendants of modern humans. The others—Atrox, Opifex, and Nano—were developed by Omni-Tek during the colonization and development of Rubi-Ka with specific purposes in mind, leaving Solitus as the baseline.
Played straight in World of Warcraft: Humans are of about average height for humanoids, somewhat bland and inexpressive in appearance. Diplomacy for getting rep faster, a bit of extra spirit, and a free trinket for escape are basically all they have gameplay-wise. In terms of pure starting stats, they quite literally are THE average, with 20/20/20/20/20 across the board.
Justified in the Phantasy Star Universe sub-series. Humans are average, but that's because they're the creator of the other races—CASTs, Newmans, and Beasts—who were developed for specific purposes. Excluding the Dewmans in Phantasy Star Portable 2 and Infinity, who are instead humans that were exposed to the SEED.
Final Fantasy Tactics Advance mostly gives humans access to classes that at least one other species can access. Not counting slight variations like our Fighter and the Bangaa Gladiator, only unique classes are Blue Mage, Ninja, and Hunter, and we can't take any of the classes that require exceptional speed, magical skill, or technological aptitude (though interestingly, our Paladin is a slightly-faster variant of the Bangaa Defender.) The sequel differentiates us a bit more with the Parivir and Seer classes, but we're still noticeably non-unique compared to, Bangaas or Vieras, the Moogles, potentially lethal Seeqsnote The Ranger class can make items act in reverse. or even the powerful Gria who can fly. (Note that unlike most examples, this does not make us useless, since a): we're better at multiclassing, and b): Ninjas can dual-wield.) There are also human-unique classes, of course, which are good enough reasons for using them. Some classes do have similarities with others and are basically just the same class with different names.
In Halo, humans, who are noted to have average projectile weapons, vehicles, shielding, and technology, compared to the advanced Covenant race, manage to win a protracted war of attrition against the Covenant, who have several species that are far more fit for combat than humans are. This becomes something of a plot point after the events of the main games: In the Cold War that follows after the war, the Sanghelli struggle to rebuild what's left of them - while humanity, which has lost every colony and has had Earth partially obliterated, is rebuilding much quicker - simply because they're average at everything, compared to the Sanghelli, who are great at one thing: war.
Justified in Fall from Heaven, where humanity is the original race in which all of the Angels had an equal hand in creating. The other races are simply humans whose ancestors who spent a lot of time with a particular Angel, whose presence caused them to become more like that Angel. For example, elves hung out with the Angel of Nature, Sucellus.
In Sword of the Stars, humanity has average industrial capacity, research ability, terraforming speed and population growth. They are almost in every respect the Jack of All Stats, except for their unusual FTL drive that makes human fleets something of a Fragile Speedster on a strategic level, and also much more vulnerable to entrenchment. Humans also have a fairly high chance of getting most of the weapons techs, unlike most other races, who tend to favor one or two lines of weapons.
In Mass Effect, humanity appears to be more or less average on all counts: compared to other galactic races, they don't have any particular physiological advantages, their level of technology is quite low, and their military is rather small. By all rights, humans should be one of the lesser civilizations of the galaxy... if not for the fact that within seventeen years of discovering Prothean artifacts on Mars, humanity went from being barely able to make it out of their biosphere to becoming a notable galactic power. While humanity as a whole may not be naturally gifted, they don't suffer from any particular shortcomings and have even demonstrated the capacity to match other species on their own terms when they fought the turians to a draw. The main advantage humans have is that we specialize on an individual rather than organizational scale and avoid Klingon Scientists Get No Respect.
Sort of straight and sort of not in the Fuzzy Knights roleplaying game. Human type fuzzies are stated to be the baseline, like in most games, but they also are a tiny minority, making up only 1% of the Fuzzy population.
In The Battle For Wesnoth, humans have no preferred terrain types (except maybe plains by virtue of no one else being exceptionally good on them), have no special preference for melee or ranged combat, have both lawful (loyalists) and chaotic (outlaws) units, and can learn many different kinds of magic without being racially focused on one specific kind.
FTL: Faster Than Light has humans being completely average in every way, without any special abilities as the other races have, but have no weaknesses either - even the one race equivalent to humans has a telepathic ability. Even the game itself notes that humans are common and boring.
Averted in Fairy Dust. Each race tends to consider itself the default. From most other race's point of view, humans are quite gifted at organising into large groups, and specially weak when it comes to mental stability.