For Mac users, I like OmniGraffle. You can build mindmaps or other hierarchies in outline form that gets rendered as a diagram as you go, which is pretty nice. You can then do whatever else you want with them too.

I also gave Curio a try recently and liked it's note taking abilities. It has a mind mapping tool that's relatively simple and keyboard driven, but not as flexible as Graffle. Curio does have some nice general note taking tools, though.