I like WinMerge but its not perfect. Emacs's ediff is also great (and slow) but I'm not using emacs currently.

Winmerge for 2-way and kdiff3 for 3-way although I'm trialing meld now there's an official windows installer.

Meld on Linux. Winmerge on Windows. I cannot find anything I like on OS X - XCode is fat as fuck

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).

Check out Kaleidoscope[0] for OS X.

[0] http://www.kaleidoscopeapp.com

$70 - no thanks...

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)

p4merge for showing 3-way diffs.