Part of the Seattle.rb Project:

cached_model - an ActiveRecord abstract model that caches records in memcached
RDoc
firebrigade_api - API for the Firebrigade service
RDoc
flog - Ruby Sadism and Pain Reporting
Rdoc
heckle - Unit Test Sadism
Rdoc
hoe - extending rake to provide full project automation.
RDoc
Hoe Down! See who's using Hoe
ImageScience - Hate RMagic? Get Science! ImageScience!
Notes
Rdoc
IMAPCleanse - IMAP automation for large lists. Automatic scanning and purging.
RDoc
mem_inspect - ObjectSpace.each_object on crack
RDoc
ograph - generates dot graphs of your objects' connections
Rdoc
RailsRemoteControl - modify and monitor a live Rails application
RDoc
rbayes - an email-focused bayesian filter
RDoc
rc-rest - a framework for writing REST API wrappers
RDoc
RingyDingy - a little boat that keeps your DRb service afloat!
RDoc
ruby2ruby - generates readable ruby from ParseTree
Rdoc
tinderbox - "I'm gonna light you on fire."
RDoc
icanhasaudio - "I is in ur computer, decodin ur music."
RDoc

Other Seattle.rb Projects hosted on rubyforge:

Rubyholic - a tool for organizing rubyists.
rubyholic.com
codeforpeople's rubyforge - rubyforge release automation.
Project Page
ZenTest - first class testing tools: autotest, unit_diff, and more.
Project Page - RDoc
RubyInline - embedded foreign language extensions.
Project Page - RDoc
ParseTree - ruby language processing tools.
Project Page - RDoc
ruby2c - ruby to C translation.
Project Page - RDoc
MetaRuby - rewriting ruby in ruby.
Project Page
ZenHacks - a grab bag of hard core ruby hackery.
Project Page