GotApi.com is almost all reference in one.
I'm used to code in various languages and libraries, and having tabs with each documentation in my firefox is really boring.
The GotApi interface have tabs, then you can have many languages opened in the same browser window/tab.
They links and allow you to browse and search in an ajaxy way many languages, to know:
- Drupal CMS
- Flickr API
- Apache Ant
- ...and the list is growing everytime..