Diffuse is a graphical tool for comparing and merging text files. It can retrieve files for comparison from Bazaar, CVS, Darcs, Git, Mercurial, Monotone, RCS, Subversion, and SVK repositories.
Probably really good, if it works on your system. I'm on OS X 10.8.4, and it crashes if I resize the window. Also, it's still listed as being on Fink at one of the source force pages (can't share links in reviews), so I went through an hour of hassle installing Fink just to find out it's not there any more.