gotAPI.com is a nice AJAX search frontend for a number of sources of technical documentation, providing a consistent and powerful way to search them all. (Thanks to John for pointing this out.)

Matching search results autocomplete as you type, and the table of contents is organized into a expandable tree view.

The currently available sources of documentation are below:

HTML

HTML at htmlhelp.com
HTML at msdn.microsoft.com
HTML at w3.org

Cascading Style Sheets (CSS)

CSS at htmlhelp.com
CSS at msdn.microsoft.com

JavaScript / HTML DOM

JavaScript/DOM at www.w3schools.com

Prototype.js at www.sergiopereira.com

XML

XSL at www.w3schools.com
XPath at www.w3schools.com

XML Schema (XSD) at www.w3schools.com

XML DOM

XML DOM at www.w3.org

C / C++ / Standard Template Library

C, C++, STL at cppreference.com

PHP

PHP at php.net

Ruby / Rails

Ruby / Rails at ruby-doc.org & rubyonrails.com
Ruby Standard Packages at ruby-doc.org

Perl

Perl Functions at aspn.activestate.com

MySQL

MySQL at mysql.com

PostgreSQL

PostgreSQL at postgresql.org

ActionScript

ActionScript at macromedia.com
ActionScript (frames) at macromedia.com

ColdFusion

ColdFusion Tags & Functions at macromedia.com

Java

J2SE v1.3.0
J2SE v1.4.2

J2SE v1.5.0
J2SE v1.5.0 less UI

J2EE

Struts
Spring Framework
Castor
Log4J
Apache RegExp

Apache Ant

Ant at ant.apache.com

DITA

DITA at oasis-open.org