dhewm 3 is a Doom 3 GPL source port. The goal of dhewm 3 is bring DOOM 3 with the help of SDL to all suitable platforms. Compared to the original version of Doom 3, dhewm3 has many bugfixes, supports EAX-like sound effects on all operating systems and hardware (via OpenAL Softs EFX support), has much better support for widescreen resolutions and has 64-bit support. Note that while the Doom 3 source code has been released under GPL, you still need to legally own the game and provide dhewm3 the game data to play.