[makmanalp]

What about vimdiff, that's already installed on most (unix-based) computers?

http://andrejk.blogspot.com/2008/04/vimdiff-howto.html

[burgerbrain]

Agreed. The great part about vimdiff is that you can easily use it to rapidly make changes and view the diff of them. It's essential for cleaning up changes before a commit for me.

[gnosis]

vimdiff is nice, but suffers from one major deficiency. It can't do three-way diffs.