Blizzard has said that Warcraft and Starcraft have been "heavily influenced" by Games Workshop's Warhammer Fantasy Battle and Warhammer 40,000 franchises.

There are quite a few differences, but some of it is so blatant (such as 90% of the Zerg concept) that arguing against it is pretty difficult.

Although I haven't checked on it, I remember hearing that Blizzard had to credit Games Workshop in the credits of one of the Starcraft games to avoid potential lawsuit. I don't recall if it was in SC1, Ghost (lol), or is included in SC2.