Danger from the Deep is a sub simulator, it's open source, and it's free. Right now it's only for Linux and Windows, but it uses SDL and OpenGL, so in theory could be ported to nearly any *NIX-based OS. The graphics are really beautiful. If you've never been in a sub, they convey the cramped, rough life that used to be a WWII sub. You'll have to have some moldy sock soup and rusty bolts nearby to simulate the smell, but the overall ambience is fantastic. DD isn't ready for primetime as a game however, as damage isn't realistic (one shot and a ship sinks immediately).