Stumbleupon Review of : http://www.pivotaltracker.com
Luvly Jubbly.
A project management system you don't have to spend more time on than the actual projects you are tracking.
It has a pretty much flat initial learning curve (see the short intro screencast) but seems to have more under the hood when needed: including Fibonacci sequence based difficulty profiling, which, when you come down to it, is pretty much essential in this day and age.
Be nice if it had nesting like todoist.com, but the tagging makes up for that.
ed (22 Sept '09): Actually, it does have a form of nesting, you can turn on tasks in the settings to further split up the primary stories.