free ways to track agile stories

a meta type of topic. what is your favorite way (free) to track agile stories (if you use agile or something similar perhaps) you are working on? spreadsheet? note cards? note app?

We use Trac -- the current iteration is set up as a milestone, and the stories are represented by tickets. We've customised it so that each ticket has an estimated number of days and the number of days actually taken, so that we can track velocity.

Thanks @giles, that looks good. I'll check it out.