Cross platform software can be difficult to debug. Having a set of virtual machines on hand that can be used for testing can be quite handy. Here are a few resources I’ve found for pre-made disk images.
VMWare Player images (seems to have much less variety):
Gentoo (didn’t find a VirtualBox image)
VirtualBox images (large variety):
VirtualBoxImages (mainly torrents, large index)
VirtualBox Images (direct download, hosted on sourceforge)
I think that should cover most open-source OSs.
As far as windows goes, one option is to roll your own and save it in its initial state. If you have access to the MSDN, most OS releases are quite readily available.