forgot all about this program! looks like it's still the best option for comparing two folders. I've been using Singlemizer (because a client already had it) and dupeGuru (because I needed dupeGuru Picture Edition, and dupeGuru is covered by the same license) when looking for duplicate files --both because they return somewhat different results. When those searches show me that there are certain folders which have a large number of duplicates, I turn to diffmerge to see what's going on. Once I've trashed the offending folder(s) I rerun the duplicate finders, and have a much more manageable file list to deal with.

The file comparison works really, really well, better than a lot of expensive tools that I have tried. I keep coming back to DiffMerge even though the UI looks so simple and boring.
I sometimes use it in combination with VisualDiffer (Mac, App Store) or ExamDiff (Windows, Freeware) to get a more traditional look.

just discovered diffmerge, and it's really great. other, pricey, comparison apps probably do this to, but I like that it lets you have a whole bunch of folder comparisons open

Really great app, it's free and for the price it's much better than other commercial diff/merge apps.