UFO: Alien Invasion

This is a squad based tactical strategy game, heavily inspired by the
X-COM PC games, but with twist. Old veterans will appreciate how the
game captures the spirit of the UFO: Enemy Unknown, the first one in
the X-COM series. It tries to combine military realism with science
fiction with the weirdness of an alien invasion. The game, however,
is not a remake or a sequel.
It runs on a modified Quake 2 game engine.