I have had some success with:

ScrumWorks (there is a free and pro version) VersionOne (paid only) Jira (paid only, needs plugins)

Banana Scrum is free and looks good. It was talked about on HackerNews awhile back.

After experimenting with lots of this software, my team has reverted to the whiteboard for Iteration Goals, and sticky notes for the iteration's tasks. We've found that you can't beat the tactile qualities of post-it notes.

Items in a software package can be easy to ignore if you don't manage it closely. But if you look at the whiteboard and see more sticky notes in the "pending" column than there should be, you instantly know something is wrong and can dive in.