Clamp is a framework for Ruby command-line applications.
Pith is the static website generator used to create this site.
ShamRack is a Ruby library that makes it easy to stub-out external HTTP services, for testing.
Representative makes it easy to create XML or JSON representations of Ruby objects.
arboreal is an ActiveRecord extension to support navigation of tree-shaped data.
(I'm no longer active on the following projects ...)
BasketCase makes Rational ClearCase ever-so-slightly less painful to use.
Selenium is a web-testing tool. I was an active contributer in the early days.