If you’re looking for a personal note-taking app, or a wiki for your team, Instiki’s the way to go, really.
I’ve never liked wikis. I didn’t like wiki syntax, and I thought they always looked really slapdash and ugly. But, I’ve seen the light. I’ve seen Instiki (found via a whole lotta nothing. It’s painfully easy to install, and even easier to use. You can use Textile, Markdown or RDoc with it, and it gives you basic formatting tips right next to the editing window, so there’s no chance of getting confused. In OS X, it even gives you a Wiki menu item, which makes using it for random meeting notes and to-do lists REALLY easy.