I've got my head in a lot of different places at once right now, thinking about Pokey's refit for the next firefighting competition.
One tool I like for keeping a bunch of related thoughts straight is Freemind, a mindmapping tool that is, as you might guess, free. It's also cross-platform (Mac OS X, Windows, Linux, or really any OS that supports Java).
The interface is quick to use because it supports simple keyboard shortcuts for every operation. Once you learn those shortcuts, you can add your thoughts quickly (sort of a mind dump I guess...)
Here's an export of my mind map so far for Pokey: