Tremulous is a free, open source game 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.
Dynamic FPS with simple base building mechanics. Human vs Aliens. Many custom servers. Strategic thinking REQUIRED! As player get money per kill, and if one die too much enemy team gains an advantage.