I just did that e-book on Scrum (shameless plug: http://www.lulu.com/product/ebook/scrummaster/18803035), and I used Trello for all the task tracking. In fact, I plugged it in the book as the best online Agile/Scrum tool I've seen so far, even providing instructions on how to use it working remotely with Scrum or Agile. I'm using it to prioritize vacation spots for our family this year, my vitamin list, some tasks I'm farming out offshore, and another book project. I also have another website idea that I'm getting ready to load up.
Things I'd like to see? 1) Linked boards. Have the same column appear as the end of one board and the beginning of another. This could allow you to have several boards with different audiences but they would all work together. 2) downloadable data. I know you guys say you want to do this, but closed data is a deal-killer for me. 3) Make it work on my iPad. Seriously. Being able to update using finger gestures on a extremely portable device would be sweet.
If you guys haven't tried it, you should. I am not a big fan of online Agile-like tools, but this rocks. Just a simple list of stuff and customizable columns to move the items around. I think the Fog Creek guys are really on to something here.