FileMerge
Being part of its developer toolset, Apple's own merge tool comes at no additional costs. It might not be the most elegant tool, but it's definitely a solid one that does the job of comparing & merging text.
Beyond Compare
Originally a product for Microsoft Windows, the Beyond Compare team has contributed a fine diff tool to the Mac platform. Like Araxis Merge and DeltaWalker, it goes beyond (pun intended) comparing simple text and also allows diffing Word and PDF contents. In its 'Pro Version', it also supports merging.
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Araxis Merge
One of the few diff tools that works with more than just text and image files, Araxis Merge lets you also compare office documents (like MS Word, Excel, Powerpoint, or ODF). It comes in standard and professional editions, and for people working on both macOS and Windows, it's great to know that a single license is valid for both platforms.
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DeltaWalker
Just like Araxis, the DeltaWalker app also lets you compare office files. However, it goes one step further by letting you compare file archives like ZIP, JAR, and TAR files. Also, if you're regularly performing comparisons on a folder basis, DeltaWalker shines with a great performance in this area. DeltaWalker supports macOS, Windows and Linux.
Kaleidoscope
Although it has changed hands (i.e. owners) multiple times, Kaleidoscope is still one of the best diff & merge tools for the Mac. Its beautiful user interface and great image diffing capabilities are what set it apart. Kaleidoscope is also available for the iPad.
Free Alternatives
In case you're looking for a free alternative to the standard FileMerge app, you should also have a look at P4Merge and DiffMerge.
Both can't compare in terms of features and user interface with their commercial competitors - but make for a valid alternative on macOS, Windows, and Linux.
Integrations
Another aspect to watch out for is integrations: before choosing your favorite tool, you should make sure that it plays nicely with the rest of your tool chain. I can already confirm that all of the mentioned tools work seamlessly at least with Tower, our own Git client.
If you don't need the power of a dedicated Diff tool application, the integrated diff views in Tower might be absolutely sufficient for you:
In case you don't know Tower: it's the Git desktop client for Mac and Windows that helps thousands of developers and designers to easily & productively work with the Git version control system. Try it 30 days for free!
Conclusion
Diff & merge apps are amongst the most underestimated tools. But a good one can be really helpful in a lot of situations. Try one of the above and see for yourself!
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