FileMerge
FileMerge comes with the Apple’s Developer Tools. When I saw it for the first time I
found it quite awkward to use. Still I think the GUI does need an overhaul. Especially I am missing some keyboard shortscuts. But it has been useful to me for a couple of times now.
Basically you click on the difference and select whether you want the left or the right version in the merge. The result is being displayed at the bottom. With some wrapper scripts you can even use it directly from subversion. Nice!