I use VoodooPad. It's lightweight, and it can be as complex or as simple as I need it to be. Failing that, TextEdit would be my go-to app. I have Pages and I have NeoOffice, but I think they're overkill for class notes. For that purpose I want something that's quick to load, quick to save, stable, and not too distracting. VoodooPad is more like a wiki than a notebook, you put in little links to other pages of your notes. The coolest thing is that VoodooPad happily exports your notes to your iPod--and the links still work on the iPod! It's also very keyboard friendly.