Second on Meld - great UI, compare folders as well as files. You can get Meld on Windows (unofficial installer-works fine) and Mac through MacPorts (meh).
Araxis Merge. It's not free, but I've never used anything as good as it.
tkdiff because (a) it installs on Linux with requiring lots of dependencies and (b) supports a -r parameter for diffing against a previous version from source control (but only for Subversion/CVS; not Git)