Pivotal Tracker has just been updated with new features.
Labels Enhancements & Saved Searches
There’s a new panel on the project page that will help you organize and keep track of related stories in your project – Labels and Searches. Open it using the View menu, or with Shift-L. On this panel, you’ll see all labels in your project, as links, with numbers indicating the number of stories that have yet to be accepted. Click on a label to see stories with that label.
You can now save any search, by clicking on the disk icon 
at the top of search results. Saved searches appear below labels, on the Labels and Searches panel.
The new panel also allows you rename or delete labels, and delete saved searches. Hover over a label or saved search, and you should see rename 
and/or delete 
buttons.
Ability to apply and remove labels to selected stories
It’s now possible to apply labels to multiple stories, using the Actions menu at the top of the project page. Select stories using the small checkboxes to the right of story titles. The same action also allows you to remove labels.
Ability to export selected stories to CSV
You can export selected stories in the project, also by using the Actions menu. One possible use of this feature is to move or copy stories from one project to another. Select some stories, export them to CSV, and import that file into another project.
API: Get stories by iteration or iteration group
We’ve made it easier to retrieve stories by iteration through the API. You can get stories based on an arbitrary iteration range, done iterations, the current iteration, or iterations in the backlog.
See the API Help page for more details and examples.