Download Aleph One: Marathon 2
Aleph One: Marathon 2 is a freeware game but this does not mean that it is not of a good quality. Actually, the game is remarkably good in terms of gameplay, even if the graphics are not exactly what you would have expected. The game is fully open source and awaits modification from fellow players.
The game is built as a sequel to the original Marathon game and you will enjoy playing it as you will have to get through a series of missions that will put your skills to the test, but you will receive good weapons to help you fight.