kaleidoscope

Perforce offers P4merge for free, and it's not limited to Perforce, it does work with any versioning system (Git, SVN, ...) , offers 2 ways diffs (for merge) and 3 ways merge (for conflicts), which is really great with Git: You see your branch version, the other branch, and the common ancestor.... once you start using 3-way diff for resolving conflicts, you can't go back to a classic diff tool!

(this screenshot shows some windows version, but it's the same UI on Mac - unless the windows specific borders)

Note: it's now known has Helix Merge Tool, but was still available as p4merge on brew a few days ago.

I started using it on Windows a few years ago with SVN, now use it on Mac for Git, and there's also a Linux version, and it's free!

PHPStorm

Another one with PHPStorm, or you have FileMerge too