I keep an online Commonplace book in the form of a wiki, using the same MediaWiki software that powers Wikipedia. It's not fully open to the public, but it's helped me a LOT when it comes to coming up with and organizing ideas, as well as recalling info about my various interests. The reason I went with a digital one is because I wanted it to be searchable :) Also, I'm a big fan of lists, so those make up a big part of the content here.
Here are some of the things I've put in mine:
* Lists of deities, sorted by religion
* Info about physical virtual pets, complete with pictures, release dates, etc
* Lists of off-brand MP3 players (non-Apple made)
* Ideas that later turn into blog posts
* A page dedicated to my favorite quotes
* Topics from my worldbuilding project
* Page with lists of different analog horror series (with links)
* lists of ARGs I'm interested in
* Code snippets and command line references
* Pictures of building ideas for Minecraft and Luanti
* Detailed reference info about each of the electronic devices I have in my possession
* lists of projects I want to try, both physical and digital