Give
DevonNote a try. It's quite powerful and not too expensive.
EagleFiler looks very interesting but I haven't tried it.
xPad isn't as powerful, but it's free, as is
Journler, which is aimed at making journals but seems like it'd work as a notetaker. I like DevonNote a lot, but it's been superseded in my workflow by Scrivener, which combines notetaking, info organizing, and writing brilliantly. I'd probably give xPad and Journler a try first, since they're free, and see if they work for you before trying demos of the paid apps. Let us know what you decide and why.