Hi,
  I just released version 0.2.0 of bond, a gem which improves irb's
autocompletion. The most notable feature of this release is custom
argument autocompletion of any method. This screencast shows this
feature in action: http://tagaholic.me/2010/05/07/screencast-of-argument-autocompletion-for-methods-in-irb.html

To install:
   gem install bond

For more:
* Source: http://github.com/cldwalker/bond
* Docs: http://tagaholic.me/bond/doc/
* Homepage: http://tagaholic.me/bond/

Gabriel