Works on Windows 2000 and all later versions.
Game data is stored alongside the .exe, so you should move ClassiCube.exe out of your downloads folder.
Only tested on OSX 10.5. It may or may not work on later versions.
You will need to make the client executable after downloading. You can do that by typing this in Terminal:
chmod +x ClassiCube
libcurl and libopenal are required. Tested on ubuntu, but should work with other distributions.
You will need to compile from source code.