I've uploaded to Flickr a couple of pictures of my current index card system that I use to track the stuff I need to get done.
In the end, after a year and a half trying several different systems, this is the one that stood the test of time: it works for me.
I would love to have something online that I could work with, specially because working with other people using index cards is not practical, but I haven't found it yet (I will try Hiveminder again just because it seems they have a IMAP interface, I'm curious about that. update: false alarm, no such feature exists, but it would be an interesting one).
One thing that I like with this index card system is the built-in expiration cycle of tasks: monday morning, I look through all the cards and I rewrite some of them, to merge related stuff. When I copy a task from a card to a new one, I add a number at the end, like (1). This is the number of times I copied the task without completing it.
It gives me a very good view about tasks that are really not that important.