Tremulous: Free, Open Source FPS Game With Elements Of RTS

Tremulous is a free, open source game for Windows, Linux and MacOSX (unofficial version for Mac OSX) based on the ioquake3 game engine, that blends a team based FPS with elements of an RTS. Players can choose from 2 unique races, aliens and humans.

Players on both teams are able to build working structures in-game like an RTS.