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.