There have been a lot of these, but here is one more.
Got API has a fast api search that can query across a lot of the APIs that you use day to day. You select the APIs that you care about, and search away.
Nice easy here are some of what I have checked now.
* CakePHP @ cakephp.org
* MySQL @ mysql.com
* Struts @ struts.apache.org
* css @ msdn.microsoft.com (ok not! just kidding)
* Prototype.js @ www.sergiopereira.com
What do you have checked?
(Via Ajaxian Blog.)