A Real Time Strategy game (RTS) created by Blizzard (the creators of Diablo and Starcraft) it has become immensely popular and has spawned several other versions, include Warcraft II, Warcraft II Battle.Net Edition, Warcraft III, Warcraft III: The Frozen Throne, and World of Warcraft, the newest in the series not yet released.
There was an attempt to clone Warcraft II, the Freecraft project. The game engine itself was a clean room reimplementation, but it required a (legal) copy of Warcraft II for textures and other artwork. It was planned to eventually reimplement the artwork as well, but the project was shutdown by Blizzard. The main reason for the shutdown was that Blizzard believed that "Freecraft" infringed on their Warcraft and Starcraft trademarks. In addition, they believed that the "clean room" that reimplemented the game engine was a little dirty. (In other words, the libre clone of the game engine was too similar to the orginal for comfort.)
With the collapse of the Freecraft project, there is no clone of Warcraft/Starcraft for Linux, and Blizzard has yet to release a version for Linux themeselves.