Classifier is a Ruby gem developed by Lucas Carlson and David Fayram II to allow Bayesian and other types of classifications, including Latent Semantic Indexing.

Handshake, currently in version 0.3.0, is an informal design-by-contract system written by Brian Guthrie in pure Ruby. It's intended to allow Ruby developers to apply simple, clear constraints to their methods and classes.

Andrzej Krzywda has put together a solid, 15 point tutorial on how to build a Rails application from the ground up using Test Driven Development (TDD) techniques. This is pure gold because so few of the books and "how to build a blog in 5 minutes" type articles bother to cover testing and Andrzej shows how to get into the habit from almost the first step.

Gibberish is a Rails plugin developed by Chris Wanstrath which provides a hassle-free text translation capability for your Rails applications. The first application to use it is the Beast forum system.

A few days ago Kirk Haines announced the release of Swiftiply, an "agnostic clustering proxy for web applications that is specifically designed to support HTTP traffic from web frameworks." In particular, it's a fast, clustering proxy that uses untraditional methods to deliver a lot of dynamism, reliability and performance. Naturally, the first target for Swiftiply's benefits is Rails, in the shape of a replacement to the mongrel_rails script (merb is also directly supported).