Our team at Wicked Loot uses Pivotal Trackers. Works fine for a team smaller than 10 if you are an agile based developer. It's easy to use and our whole team uses it to manage our tasks for the week. Their velocity system, which estimates how much work you can get done in a week, requires you to stay on top of task tracking, otherwise it doesn't work. It can't quite parse through a lot of tasks, so if your project gets bigger, you might want to opt for a system that handles project tracking and bug tracking all in one, like Redmine.

I use Assembla.com. I think it's great for what you're looking for. We've been using it for around 6 months. The version we use is $190, but you can do a free one with just 3 members I think. Even that price is not too bad if you're serious about it and can't find something else just as good for free. I'll attach a screenshot of my workspace in Assembla.

I've been using Scrumwise for school projects. Really easy to work with and very visual. I think it's perfect for a small project. Although I haven't got much experience with other online SCRUM tools, so can't really make a good comparison. Scrumwise appealed most to our team when we had to choose between SCRUM tools.

- I now see someone necro'ed this, err

I personaly use Todolist. No scrum. It s a free program.

-I was mainly interested in the task hierarchy, like a tree.

-There is a timer. So I use it to evaluate and improve my own work.

-There is a progress percentage based on the hierarchy.

Just rekindled my flame with Trello :)