A while back I worked with a gentleman who had setup a personal phpBB-based forum on his web site to use as a knowledgebase. Whenever he came across some tricky technical tidbits or arcane syntax, he would make a note of it in the phpBB forum for later use. It had never occured to me to do this, but once he mentioned it I thought it was a great idea and set one up for myself.
I set mine up about 8 months ago, plus or minus. Unfortunately I have not used it nearly as much as I would like to. The main factor holding me back is the structured format of a web forum. phpBB is great free software, but it's not intended to be used as a knowledgebase. Setting up new forums and topics to keep ideas organized quickly became too much overhead for me to bother with.
Recently I have moved over to a wiki-based knowledge base and I am getting much better productivity out of it. While it's not exactly a groundbreaking, earth shattering idea, I do think that wikis can be great personal productivity tools for those technically inclined enough not to be intimidated by the prospect of setting one up.
In addition to using the wiki as a repository of technical tidbits, I also use mine for keeping track of projects I'm working on, to-do lists, people I've been talking with, etc. Most wikis are based around the concept of easy group collaboration, but the low barrier to entry to adding and linking content also has useful implications even if the wiki is private and only used by a single person.