They vary wildly from far too technical for clients to use (Trac, Redmine); to too rigid to handle fast moving tasks (Trac and Redmine again, ActiveCollab); to too simple for complex/long-lifed/complex tasks (Basecamp, Trello).
FeatureZen is a pretty new addition to the stack (made by someone here at HN) and it's showing promise, but it's still very young.
I also came across sprint.ly recently, and thought that it looked interesting. I've put much thought into making a task tracker that fits "the real world" (whatever that may be), but every time I think too hard about it, I baulk at the size of the task.
In short; find something that suits your style, use it. If you need something different, don't be afraid of using two things for different situations. It becomes a bit of a nightmare to manage, but if it's just you, then it's easier to keep on top of it.