I'm using
Tiddlywiki for three projects right now, although they theoretically could all be part of the same wiki and kept separate by tagging and naming conventions. I'm finding it useful but I'm probably not using all the features available--use it mostly as a notebook with a page for every day, but it's better than my physical notebook because I can search all the text for what I am trying to remember.
Mediawiki might be overkill for a personal system, but maybe you have a lot of things going on in your life :-P Ideally if you wanted something accessible from anywhere (with a computer, that is) you could store a Tiddlywiki in your Dropbox and sync it everywhere, or you could install something lighter than Mediawiki on your server and be able to add/edit things from your smartphone (if you have one).
I'm sure there are lots of thoughts to be had on this subject.