For my personal notes, I usually rely on good old pen and paper. Unfortunately, organizing and remembering to act on my notes isn't very efficient with a bunch of scribbles on a paper notepad or index cards. Also, there's that possibility that my notepad or index cards can get lost. A good solution is to store them electronically.
One good solution is to use a service that lets you save your notes on the interwebs, such as Springpad. This web service lets you save info and ideas easily, organizes those stuff by category, and helps you act on them via reminders. What's neat is that we can access our data via a computer or mobile phone. And it's free!
Springpad [web link]