When Bungie announced the end of their relationship with the Halo series last year, it came as a shock to many. They weren't always the Halo studio, though; years earlier they were known for a Mac series called Marathon. It was a sci-fi shooter trilogy with a lot more in common with Halo than you might think—and now you can play the whole thing for free on your PC.
The Marathon Open Source Project has finally released their mangum opus, a totally free, multiplatform collection that includes Marathon, Marathon 2: Durandal, and Marathon Infinity, as well as Aleph One, the engine on which all three games now run. The games and engine work on Windows, Mac and Linux, so no matter which type of system you own, you can experience these awesome retro titles. If you can get past the oldschool style, you'll discover challenging gameplay and a plot as rich as Halo's.