The similarities between StarCraft and Warhammer 40,000 are often cited as the source for their comparisons, in truth both were created independent of each other. They did however draw from the same sources, like Heinlein and Dune.
It would be more accurate to say that, while WH40K fans frequently accuse Starcraft of ripping their setting off wholesale, they're both much more influenced by Starship Troopers, published in 1959, and both took advantage of the "space marines against insect-like aliens" motif seen in Alien and Aliens, released in 1979 and 1986 respectively.
Much confusion has resulted from Blizzard Entertainment's popular Warcraft franchise being heavily "inspired" by Games Workshop's comparatively obscure Warhammer tabletop war game. A new round of this ensued when a WarhammerMMORPG was announced with, unsurprisingly, more than a few similarities to the mega-popular World of Warcraft prompting this Penny Arcade strip.
The similarities are so close that there are persistent unfounded rumors that Warcraft started as a Warhammer game, but Blizzard couldn't gain Games Workshop's support and dropped the IP (and Warcraft has moved quite far away from Warhammer Fantasy Battles a while now).
Naturally, both drew on others as well. Warhammer Fantasy took from Tolkien and D&D.
There are plenty of similarities between the Warhammer Fantasy setting (published in the 1980s) and Tekumel (published in the 1970s), as well; both exist in a pocket universe, have two moons, are caught between the usual Forces of Chaos and Order, have long-lasting Empires with complicated in-story histories, a salad of races drawn from the gene-stock of the external universe. That said, they've gone different directions.
They share only tertiary characteristics. Warhammer Fantasy is a fantastic take on XVI-century Europe and visceral dark fantasy with heavy horror undertones and strongly relying on Ruleof Cool, while 'Tekumel' is tolkienisque approach to high-fantasy Mesopotamian and Mesoamerican cultures based on actual scientific research. They have hardly anything in common.
They also drew heavily from fellow Brit Michael Moorcock, who basically invented all that Chaos stuff Warhammer uses so willingly, up to its very symbol — the eight-arrowpoint Chaos Star.
One of the odder names in the 40k lore is that of the Primarch Lion El'johnson, of the Dark Angels chapter of the Space Marines. The real Lionel Johnson was a poet, whose most recognized work — The Dark Angel — is all about the torment of someone both Catholic and gay. Given that the DA's backstory is all about how half of their brotherhood fell to Chaos, and that the Imperium is Super-Catholicism on steroids (complete with not one but THREE Inquisitions), you do the math.
Yeah, that's called Genius Bonus. Both Warhammer universes have good share of them being chock full of culture references.
Speaking of Dungeons & Dragons...the displacer beast (a catlike monster with tentacles sprouting out of its back) is based off of an alien called Coeurl from A.E. Van Vogt's short story Black Destroyer (later compiled into the novel Voyage of the Space Beagle). So are the "coeurl" enemies in the various Final Fantasy games (although they've generally got elongated facial barbels rather than back-tentacles), and Mughi from Dirty Pair. Perhaps ironically, the displacer beast is not included in the open game content.
The illithid are also counted as "product identity" and you can get sued for using them... Gygax got the idea from a creature on the cover of Brian Lumley's The Burrowers Beneath, which was, needless to say, inspired by H.P. Lovecraft's most famous character, Cthulhu.
Likewise, the D&D "frost worm" is identical to a wormlike monster fought by Conan in his original adventure novels — specifically the short story "The Lair of the Ice Worm" by L. Sprague de Camp and Lin Carter. The remorhaz, as evidenced by its name — the original Conan Ice Worms were also known as Remora — draws inspiration from the same source.
Xill are based of of Van Vogt's Ixtl (right down to the very similar name), which can be found in the same anthology as Coeurl.
Some 4th edition versions of the vampire that do not die in sunlight, including the class, have been accused of ripping of Twilight, even though Dracula didn't die in sunlight either.
Most folk vampires were capable of being active in the daylight. They were simply unable to use their powers or were weakened.
Tarrasque, often considered a D&D invention is a medieval mythical beast. It had to be pretty popular if there was even a type of cannon named after it but simply wasn;t used as often by fantasy authors and griffons and dragons. Ditto for catoblepas.
Some newer gamers complain that Magic: The Gathering rips off Pokémon and Yu-Gi-Oh!. Magic was first printed in 1993; Pokémon in 1996, and Yu-Gi-Oh! in 1999.
And even Magic was not the first Trading Card Game; a baseball card company published rules to a game you could play with baseball cards...in 1904.
And even better in the time when Yu-Gi-Oh! was in idea form called Magic and Wizards (1990-1996), the original monsters were already created (thus being older in design, but younger in published card form).
First modern wargame (i.e. close simulation of a real battle with set rules of conduct) was "A Tactical Wargame" designed by Lieutenant Georg von Reiswitz in 1812. It was actually not a pastime but an educational tool for officers. It also featured modular board (that looked much like modern "Carcassonne": http://www.spiegel.de/fotostrecke/fotostrecke-42723-2.html), arbiter (gamemaster), multiplayer mode (up to 10 players) and both constant (tables) and random (dice) elements.
