IHelp lets you view the ri documentation of an object from Ruby. Perfect for irb.

sudo gem install ihelp

"Hmm, how did String#center work again?"

> String.help:center

"What is this object and what can i do with it?"

> object.help

"Someone told me that I should look up Mutex#synchronize... but I don't have that loaded."

> help "Mutex#synchronize"

Now with Ruby 1.8.5 compatability fixes, an emacs renderer from rubikitch and RubyToRuby support for viewing the source of a method.