TiddlyWiki has been around for about four years now, and I heard of it a couple of times, but I only just recently got around to using it regularly. I decided I needed a decent local home page for whenever my browser started up; instead of fetching a page from the Internet (most likely Google), why not just have all the basic stuff I need, like reminders and such?
Out of the box, TiddlyWiki looks simple to use. If you've used a wiki in any form before, you can get the hang of it pretty quickly. If not, there are several tutorials out there. Being somewhat obsessive-compulsive, it took me a while to customize it to my tastes; that, unfortunately, was a little harder to figure out, but became easier as I got the hang of it.
The default TiddlyWiki installation is a little basic, so I've had to spruce mine up with a couple of plugins. I installed a theme from TiddlyThemes, the ForEachTiddler plugin from abegoExtensions, and the Calendar plugin from TiddlyTools.