You're looking for Notational Velocity. It's free, open source, minimalistic, keyboard-centric, and extremely fast. I use it all day, every day.
http://notational.net/screenshots.html
My life depends on Notational Velocity. It's the fastest note taking system I've ever used. The killer feature is the ability to go from search to edit instantly. It's so good, that I'd consider buying a Mac only because of it. I recommend NValt, a more updated fork.
Notational is so badass. I'm going to give OneNote a hard look when I have some time but I'm so used to Notational that I might just stick with it. I would like to see some additional formatting options in NV but the speed and UX are unbeatable.
Thanks for this. I'm only a few weeks in on OSX and nvALT is just what I was looking for. I also hooked it up with iA Writer, which is only $5 at the moment.
I was looking for a personal wiki-type thing on OSX, but this is a fantastic set-up for me.
It doesn't quite do images, since it's text only. nvALT is a fork of the project with extra features. Its image support might fit your needs: http://brettterpstra.com/2012/09/27/quick-tip-images-in-nval...