Sure, this ias the list as it stands today. More APIs are being developed as I speak.
Basecamp Project Management
ChartBeat Web Analytics
Clock
Email
Get Satisfaction Customer Service
Google Analytics Web Analytics
Highrise CRM
MailChimp Email Marketing
Pingdom Server Monitoring
Prefinery Beta Management
Text Custom Alerts and Messages
Twitter Social Media
Zendesk Customer Service
Edited for formatting.
Edit 2. Removed Calendar from the list, it's not quite ready yet.