Got API
I used the Sun Java API Docs online when I used to code in Java: http://java.sun.com/javase/6/docs/api/
And the for Ruby, Ruby Brain is a nice place – http://www.rubybrain.com/ and http://www.railsbrain.com/
Even for DB, the MySQL API docs are good: http://dev.mysql.com/doc/refman/5.1/en/index.html
But today I was introduced to Got API! You get most of the stuff you are looking for, all in one place. They have CSS/XML/Javascript, even the famous AJAX libraries, C++, Java, Ruby/Rails… you name it! And a nice interface too.
Check it out: http://www.gotapi.com