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
If you are planning to go commercial with your open source product you might consider our Parabuild. It allows you to run a set of builds on multiple platforms and machines in parallel. The build will success only if all platform-specific builds success.