MyThoughts for Mac
At first I wasn't sure about MyThoughts for Mac, from Mode de Vie Software - it seemed a bit simple (particulary after using iMindMap). However, I've come to really like it. It's had a couple of updates since I first started using it, and it's simplicity is actually really refreshing (and it turns out being simple doesn't mean it's not feature-rich enough). Of course, it's Mac only. For $49 it's got a pretty good feature set, and as a native Mac OS X application, it performs nicely.
