I've always been pretty happy with Diffmerge<p><a href="https://sourcegear.com/diffmerge/webhelp/sec__inst__mac.html" rel="nofollow">https://sourcegear.com/diffmerge/webhelp/sec__inst__mac.html</a><p>Instructions for commanline use are pretty easy to find as well as setting it up as the default diff toll for git and svn
I like KDiff3 for osx.<p><a href="http://kdiff3.sourceforge.net/" rel="nofollow">http://kdiff3.sourceforge.net/</a><p>Ugly, but just works :)