Start a blog. This way, not only you benefit, but so do others who may have the same problem. There are also combination blog/wiki systems, ranging from Drupal to Hiki. Also, consider that having your notes on a host (with regular back ups) will ensure that they survive you dropping your laptop down a flight of concrete stairs, and that they will be available even when you're away from your primary working machine.
Oh, and some people really like mind maps for brainstorming and such, but I'm not sure that they're that valuable for long-term note taking/storage.
UPDATE: If you don't really want to run your own site, or a blog is too much of a time investment, another option is to start posting on RefactorMyCode or Snipplr. You get a searchable database of your code snippets, plus perhaps people will comment on your code and suggest improvements. The "Code reviews on the web" thread might have more ideas over time.