Most of my software will be written in Python, which itself is cross-platform. My software is primarily for Linux/*BSD/unix, it shouldn't be too difficult to install on Mac OS X, if you are familiar with the terminal.

On Windows, there are quite many things that may not work as easily. Users familiar with the terminal on Mac OS X/Linux/unix could use Cygwin. Notice: Don't throw away the installer, you'll need it to install dependencies of whatever you might install in the future.

List of larger software projects

List of testing platforms (mostly unix)

Tell me if you have any suggestions for a zero-cost OS that should run on a VirtualBox and should run POSIX software (native, emulation, compatibility layer, etc).

You may also send me patches and such to improve portability (even for any system on the list), but I'd like to know what credits you want, and what you don't want.