You can get reasonably far with Trello, if all you're doing is item tracking. If you're trying to do anything more than a free-for-all workflow, or wanting to do reporting, it's less good.
Thanks for the link to that -- I've been looking for something like this too!
+1 on Trello. I'm a Scrum Master and we use it in our current team (~10 devs). I miss the reporting JIRA has tbh, but we get along just fine using workarounds (estimating in cards, etc).
Good luck!
Why not use pivotal tracker then? It is trello + estimation.
Ohh, I didn't know about this - thanks for the heads up!