Back at Datastream we used Vault as a replacement for for Visual SourceSafe. It was a great tool. (Not living (completely) in Microsoft land anymore, I now use Subversion for all source control.) Bundled with Vault was a diff tool. It’s apparently grown up into an nice little app.
SourceGear just released it’s diff and merging tool, aptly named DiffMerge for free. It’s available for windows, Mac, and Linux. In fact, depending on the API, I may try to integrate it with TextMate’s Subversion bundle to finally get a good visual diff engine on the Mac.