The brainiacs over at Lullabot recently unveiled api.lullabot.com, their version of Drupal.org's API documentation site. While on the outset, it seems similar, there are some nice features of the Lullabot site that make it very worthwhile to use.
First and foremost, in addition to documentation for Drupal core, they also added the documentation for a large number of popular Drupal modules, including Views, CCK, and Ubercart.
Some of the other nice features include a handy bookmarklet that you can drag to your browser's toolbar for quick searching of the site, quick URLs of the form "api.lullabot.com/hook_form_alter" (to help avoid searches if you know exactly what you're looking for), and a smarter 404 page that actually performs a search for based on the URL you entered.
All-in-all, it's a great tool to have in your Drupal developer toolbox.