This summer you can choose from a feast of Force.com APIs currently in Developer Preview:

• The Apex REST API lets you surface Apex methods as REST-based web services (webinar signup) Check out Sandeep's blog post about Apex REST, which includes a nice code sample.
• The Chatter REST API provides a nice, clean, REST API for Chatter (webinar signup) Quinton provides a little more detail on this API in his blog post and you can get even more, including docs, by going to the Chatter API resource page.
• The Streaming API delivers HTTP push technology (webinar signup). Check out Pat's description of the Streaming API, where he gives one great use case – polling, describing the Streaming API as "a more elegant, efficient, scalable alternative".  

This is a smörgåsbord of new technology.  Apex REST will let you write killer integrations, easily.  Chatter REST will let you take Chatter to mobile devices and other apps in a much more intuitive fashion.  And Streaming API will bring push technology to your apps. 

Next steps – sign up for the webinars (see the webinar signup links above).  It's okay to sign up for all three 🙂

Note that these APIs are all entering developer preview.  They're not for production – and of course they may change.  Details of the developer previews will be made available during the webinars.  

Enjoy your summer.