The built in Xcode Documentation browser is okay, and the stuff Apple does to bring some bits of documentation lookup into the code in IDE is not bad. However, until the full documentation browser is it's own app, I don't think it'll ever reach its full potential. One day I saw Peter Hosey use OmniWeb and do a brief run through of what he did to browse documentation.
Since then, I've only made a couple tweaks to this, but I am surprised more people don't use a standalone documentation browser app like this because it's so easy to separate from Xcode and command tab over to and find what your looking for fast.
Download OmniWeb ( http://www.omnigroup.com/products/omniweb/ )
Here are some of the links I have (you'll need to drag them to your bookmarks bar for them to work)
Foundation
AppKit
Core Foundation
Developer Tools
Core Graphics
Core Services
What's New in Mac OS X 10.6 Notes
My Doc/Code Search Templates
ADC Search (Keyword: adc@) http://developer.apple.com/search/index.php?Search=%@
Google ADC Search (Keyword: gadc@) http://www.google.com/search?q=%@+site:developer.apple.com
GoogleCode Search (Keyword: gc@) http://google.com/codesearch?q=%@&hl=en&btnG=Search+Code