I've always seen mediawiki as a great gift to the universe - it's an amazing way to collate knowledge from disparate sources and make it available to the world.
It wasn't really until I had a chat with a co-worker (a guy for whom I have a hell of a lot of respect) that I really started to consider other applications. I must confess that I feel a bit dull for not considering the personal use of a wiki a long time ago, especially given my frequent assertions that I have no memory, that I'm not detail oriented, etc. The fact is, a personal wiki can be a great replacement for a brain.
Of course, the inner geek in me loves the notions of documenting, templating, and categorizing to my heart's content; silly little activities from which I derive a perverse pleasure. But really, the wiki serves a much more valuable purpose - it's a living document whose knowledge expands as does my own.
At any rate, once I realized the immediate and practical benefit to having such a device, I installed it on coltrane (which, incidentally, is the machine to which you are now connected) and had at it. I've not gotten very far, but, you know, you can always stop by and take a look.