VoodooPad is a really interesting application for Mac OS X users. As stand-alone applications go I don't know any good comparisons. But when you compare it to web applications it's easy to say, "Oh, it's a wiki." But really, it's a personal, one-user wiki, written as a fat client instead of a web application, with a few extra features thrown in for good measure.
In working with it, I've create a couple of primary pages, including Work, Personal, and Contacts, and I use those quite a bit. It's a good tool for taking meeting notes, and from time to time the ability to create a sketch on the fly is very helpful. Beyond those features VoodooPad also has some cool drag-and-drop support, but I use it primarily for the wiki functionality.
The crazy thing is that right before I learned of this product I thought out loud "Gosh, it sure would be nice if I could have a personal wiki on my laptop", and a friend of mine told me how he had a wiki set up at home for just that purpose. Shortly after that this product came out.
The good people at Flying Meat offer a free trial of VoodooPad, and I definitely recommend taking it out for a spin.