I have an Open Source app and I have it working on Windows, Linux and Macintosh ( it\'s in C++ and built with gcc ). I\'ve only tested it on a few different flavors of Linux so
From a software point of view, there's also buildbot (sourceforge project site), which can be used to set up your own build/continous integration server.
This was suggested and considered to be used for gcc development (as mentioned on the gcc compile farm wiki page posted above).