Tracker is a free, award winning, agile project management tool that enables real time collaboration around a shared, prioritized backlog.

- http://www.pivotaltracker.com/ 

On top of that, its a refreshing break from our previous project management tool,  Target Process.  While Target Process is no doubt a powerful, robust, agile project management solution; it was a bit much sometimes.  Pivotal Tracker is more asthetically pleasing as well as easy to use. 

Unfortunately, there wasn't a lot of help out there for integrating our .Net websites with their API.  The Pivotal Tracker API integration page does feature an exmple by Michael McKerlie here.  This was a decent start, so I've added to it and in turn answered some of the questions posted in his comments.  Behold, my first contribution to Google's Open Source project hosting - http://code.google.com/p/pivotal-tracker-api/.  Here you will find some great wiki pages on how to integrate Pivotal Tracker API for .Net into your solution as well as planned features.  

We currently use the API to integrate our user Feedback form with our Pivotal project.  This way, when a user submits feedback, it enters our project queue as a bug so that we have to take action on it.  It seems to work good in our enviornment, but thats probably because we have closed channel of users - I'm not sure I would recommend giving a public site this functionality.  Opening your Project Management queue to spam-bots would probably be a bad thing.

Two things I look forward to adding in the near future include examples for using with Asp.Net MVC and the ability to Export project stories during a build (NUnit) for backup.  Check back here or the project site often for updates.