Growly Notes is totally free (and very close to the look of oneNote)
However, nothing is as good as oneNote (if that is what you like). You can install OneNote 2007 in OSX using crossover (basically 'wine'). This doesn't work with oneNote 2010, only 2007. It starts quickly and works perfectly. The only problem I can see is that hot linking crashes oneNote.
If you do try to install, look for the oneNote 2007 custom setup in crossover, if during setup it complains about fonts etc... just keep going with the install, and skip and problems. OneNote 2007 will run fine under OSX 10.6
If you are looking for something a little different, but just as addictive to use as oneNote, I would recommend trying a program called Scrivener. It is one of the few programs I've found to be perfect for writing & organising my thoughts.
You mentioned freemind (mind mapping), I would totally recommend a program called Vue, it's free too, very slick, and not as linear as freemind, also allows you to add notes, keywords, layers etc.. to organise your data.
I've used all of the other suggested apps (evernote, yojimbo, etc..) I do like them, and use them. But I hope that I have highlighted a few very useful/nice apps, that most people don't know about.