From the category archives:

API

Bitbucket Integration

by Niki on January 24, 2012

GitHub Integration

by Niki on January 10, 2012

Campfire Integration

by Niki on December 20, 2011

HipChat Integration

by Niki on December 7, 2011

New REST API is live

by Nate on February 28, 2011

Introducing the Zen API

by Nate on August 3, 2009