Kanbanery iPhone App - a free application that allows view-only access to your Kanbanery boards. Later versions will allow full access to all board features.
Janbanery-Shell - Kanbanery in your shell!
This one doesn't use the API, but it's kind of fun and might be useful to some: a bookmarklet to resize your Kanbanery columns
Kanbfire - Add comments to Campfire from inside Kanbanery
Kanbanery Feedback - is a set of tools for adding a modal feedback/bug report tool to your own Ruby, PHP, or Python app that creates a ticket in Kanbanery
Pivotal Tracker to Kanbanery importer - this doesn't use the API, but it's a useful script for parsing Pivotal Tracker's exported data to Kanbanery's csv import format