Common fonts for the spread of browsers and platforms?

So what’s the most common for:

linux
windows
mac

I should really account for non-windows users and those that don’t have a particular font, that’s the only wanring i get in my css – it validates fine, but I always get use a generic font family. I guess courier is the most common one, or do most people not care for other platforms?

EDIT: Nevermind