Danger from the Deep (known as dangerdeep or DftD) is a free (as in free speech), Open Source World War II german submarine simulator. The program and source code is available under the GPL license and most of the artwork/data is released under a Creative Commons license (Attribution/NonCommercial/Non-Derivative works).
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.