ProcPile is a pure Ruby distributed computing cluster environment. Its goal is to allow fast prototyping on smaller datasets, especially those where serialization and deserialization are minor expenses relative to the core processing task. Once the algorithms are settled, you should be able to scale out to full production usage. At that point, you can replace the core processes with Ruby modules written in C (or Inline::C), or move the design to a more "grown-up" clustering environment.
Momomoto is an object-relational mapper written in Ruby for PostgreSQL. It features support for combined primary keys, support for stored procedures, and support for schemas. Metadata for tables, views, and stored procedures are fetched directly from the database to keep declaring classes as simple as possible.
BitNami RubyStack provides a fast, easy way to develop and deploy Ruby on Rails applications. It includes Ruby, Subversion, MySQL, SQLite, ImageMagick, and several Ruby Gems, and will optionally install Apache 2.2 with rewrite and proxy support. It supports Windows, Linux, and OS X, so you can share the same Rails environment on multiple platforms.
ActiveShipping is a unified Ruby API (quite useful on Rails) for shipping information from multiple carriers. It calculates shipping, handling, finds the best price from multiple sources, etc. It is primarily focused on rate requests, but may be extended to cover other aspects of shipping as well. Currently, it includes UPS and USPS modules, and modules for Fedex and DHL are coming soon.
Einarc is a unified storage RAID command line interface and an API that provides management for various hardware/software RAID devices, uniting them all in a single paradigm. Einarc works as a translator that makes it possible for a user to control all these devices using simple terms like "physical disc", "logical disc", "adapter", etc., while transparently converting these requests to proprietary RAID paradigms.