Danger from the Deep (known as dangerdeep or DftD) is a free (as in free speech), Open Source World War II german submarine simulator.
DftD is currently being developed on Linux (i386 and AMD64) and Windows. There are binarys available for Linux (i386 and amd64), Windows (32bit) and there are some old packages for MacOSX. Danger from the Deep makes use of SDL/OpenGL and thus should be portable to other operating systems or platforms.
This game is planned as tactical simulation. It’s current state is ALPHA, but it is playable.