A mention from a colleague made me look up Pivotal Tracker. It looks like a really nice tool for planning Scrums. We are using Version 1 on our current project, and I hate that beast. In comparison, Pivotal Tracker’s clean, usable interface looks really great.

I created a small test project to check it out. It is a somewhat opinionated software, and may not do some of the things that you expect it to do. For example, by default, there is no option to break down a story into tasks, or to assign points to bugs and technical tasks, but you can change that setting on a per-project basis if you want to. Check out their help section for other things it can or cannot do – Pivotal Tracker help.
And it is free with no limitations! What more can you ask for, go check it out today.
Another good write up on Pivotal Tracker –
Helping web developers and operations bridge the deployment gap
Thoughtworks Mingle vs. Pivotal Labs Tracker.
Did you get chance to check out WRAP (WoodRanch Agile Projects) at http://www.agilewrap.com.
WRAP has most of the features you mentioned in the blog post – breaking epic into multiple levels of smaller stories, breaking story into tasks, assigning and tracking defects for a story, tracking points for stories and todo hours for tasks/defects. WRAP has strong requirements management including product backlog management. Many charts like iteration/release burndowns, story burn ups, velocity charts, defects cumulative etc help track progress of project easily.
WRAP is popular for its ease-of-use, powerful historical reporting and competitive pricing (check out pricing at http://www.agilewrap.com/pricing.html ). And best of all, they offer 5 Users Free with no feature stripping upon sign up. You can use your 5 users FREE licenses as long as you want