A few months ago the primary project to which I'm assigned espoused an Agile/XP approach for development... well, a heavily modified Agile/XP approach as there is only one developer (me) working on features/tasks/etc that come from a team of people, funneled through a project manager of sorts before they get to me. Because of the unique relationship we've enjoyed with the client, managing the project has been an interesting challenge since client users are involved closely with the project and are sometimes responsible for tasks such as testing.
We've moved between a few task tracking/project management systems in the process, trying to find software that fit our needs, the latest of which is VersionOne. The client actually came across VersionOne, which I'd only heard of in passing before that point, and asked if it would be a viable system to use during development. After a quick glance by several of our team members, we agreed that the system looked pretty good and was unique in that it was designed with Agile in mind so it supported the Agile/XP workflow in a manner than other ticketing systems didn't. So, over the course of a couple days we migrated to VersionOne and started using it.