How far does your "other OSses" go? VMS? OS/390? VOS? QNX? Or is it just "Unices" and "Windows"? There's no portable way of finding out the free "disk space" (which is a misnomer, what people actually mean is free space available in a file system), at one level you've got to switch on the OS.

Having said that, I'd go for 'df', which is available on all Unices, and has been ported to non-Unix systems like Windows as well. Many Unix tools have been ported to systems outside of Windows/Unix as well, including df, although I won't be able to give you a list of the platforms df runs on.

Abigail