We released the first version of the Pivotal Tracker API a couple of months ago, and already there is a growing list of useful 3rd party tools out there:
Pivotal Tracker for Ruby
This is a ruby wrapper for the API, written by our friends at Mobile Commons.
http://mcommons.com/developers/pivotal-tracker-for-ruby
Email Integration
Also from Mobile Commons, this tool allows your to automatically create Tracker stories from inbound emails.
http://mcommons.com/developers/pivotal-tracker-email-integration
GitHub Post Receive Hook
This is web service that you run on your own server, written by Chris Bailey. It automatically updates stories in your project(s) based on GitHub commit comments:
http://codeintensity.blogspot.com/2008/12/github-post-receive-hook-for-pivotal.html
Pickler
Synchronize stories in Pivotal Tracker with Cucumber features. If you haven't heard of it yet, Cucumber is a really cool app that can execute plain-text documents as automated functional tests.
http://www.github.com/tpope/pickler/tree/master
More Tools
There are more, most of them live in GitHub:
http://www.github.com/search?q=pivotal+tracker
If you have written something you'd like to share, or know of other useful tools, let us know! Also, we're planning on making the API better, and would love your feedback.
Thanks for the shout out on the email integration. Your post inspired me to hack out a new feature I've been wanting to do. You can now automatically assign a story to someone by cc'ing them on the email.
This is the use-case: I get an exception notification email that really should be looked at by one of our developers. I simply forward the email to tracker+bug@example.com and CC the developer. Usually I'll change the subject line to be a proper story title and add my notes in the body. And it's done!