Documentation is essential for software development. I'm always interested when someone presents a new way to navigate documentation. I remember when Fileability came out with Ingredients. Sadly, Ingredients doesn't seem to work on Lion.
Nate told me about this new documentation viewer called Dash, from the Mac App Store. It works fast, can be accessible with a hotkey, and has support for "snippets" (kind of like TextExpander) which can sync over DropBox.
It also includes support for custom documentation sets, and has a growing library of one-click-install docsets.
It's free at the time of this writing, and I recommend it.