Danger from the Deep (known as dangerdeep or DftD) is a free, 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.
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.
The Danger from the Deep team are proud to announce that version 0.3.0 (ALPHA) is now available for download. New features include:
* Updated to OpenGL2.0/GLSL1.1 for lots of eye-candy
* New user-manual
* All the old shaders have now been replaced (Need a GLSL1.1 compliant card to enabled the shaders)
* 4 new uboat types, the type IIA, IIB, IIC, IID
* New skins for all the above uboats
* The "Captain's cabin" screen featuring the log book and several other new screens
* A new Tribal class destroyer with 5 new skins
* Fixed texture mappings on all models
* Water now reflects per-pixel thanks to the new shaders
* Fixed translations and rendering
* New font for all in-game text
* Improved night sky
* Periscope now blurs when wet
* All models are now capable of having moving parts (rotating turrets, elevating guns, etc...)
* Hall of fame now works
* Code is now multithreaded
* A new sound system has been added
* Improved torpedo management screen
* New basic "torpedo-cam" added
* Distance haze was improved for daylight rendering
* Water now uses geo clipmaps
* New water splashes on impact
* VBO and FBOs are now used
* Tons of bugfixes
http://dangerdeep.sourceforge.net/index.html