Blob Wars is the story of a blob named Bob, who's trying to save his race from certain destruction from the Aliens. It's a simple 2d Platform game written in SDL, so it runs on Windows, Linux, and MacOS (probably others as well). It's also the most polished FLOSS game I've ever seen.
The level design is simple, with levels unlocking as you clear others, sometimes branching into multiple paths you can take, and each level includes some things you can't access until you get the Rebreather (allows you to swim indefinitely) or the Jetpack.
There are a variety of weapons, each providing different strategies. Each weapon gives you unlimited ammunition, however, you can only have one weapon at a time, so sometimes you have to be careful not to pick up an unwanted weapon drop.
This is just a great freeware game, and I'd seriously suggest you pick it up. Anyone interested in game development might also want to consider the sequel, which is in 3D and in development at the same site.