I use them for anything I would use for taking extended notes. Kindles don't have good note-taking capabilities, so I use my N900 for taking notes on books I am reading on the Kindle. I take notes about regular books (you know, that use paper) too, and other things. When I went on a cruise in Europe not that long ago, I kept notes on the ports we were going to. Another time, I kept notes on the people I've known -- that's a big project, even for an introvert! And I kept notes on the books that influenced my thinking. And I kept notes on sites about online learning, since that's an interest of mine.

I used Mindmeister for some of that stuff, Freemind in Linux for other stuff, and I'm starting to use Zim. The nice thing about Mindmeister is that it's permanently available.

A lot of notes I make are thrown out. It is just something to help me organize my messy thinking.