We used Kdiff3 with the Mercurial DSCM (Distributed Source Control Management) system aka hg. This is free, works on Mac OS X and Windows (XP for user - don't use Vista)
Requirements:
1. not sure about vista or 10.4
2. Not sure
3. Free
Home page:
http://kdiff3.sourceforge.net/
sorry about lack of information. We use Xcode with Mercurial now.
Cheers,
Rick