Danger from the Deep
Danger from the deep (aka dangerdeep) is a Free / Open Source World War II german submarine simulation. It is currently available for Linux/i386 and Windows, but since it uses SDL/OpenGL it should be portable to other operating systems or platforms. Read more hot
omron++ is the simplistic peace-endowing warfare simulation.
OpenSSN is a modern submarine simulation that tries to balance fun, realism, and simplicity. Players are able to explore, detect ships, and manipulate sonar.