RSpec: I’m on the RSpec core team.
Ripple: I’m on the Ripple core team.
VCR: Ruby gem to record your test suite’s HTTP interactions and replay them during future test runs for fast, deterministic, accurate tests. Featured on:
Fractal Composer: Java library to algorithmically generate fractal compositions using self-similarity based on an initial seed.
Mail Safe: Ruby gem to prevent your ActionMailer emails from escaping into the wild during development. Featured on ruby 5.
Test Benchmarker: A tool for benchmarking ruby Test::Unit tests.
NullDB: An ActiveRecord null database adapter for greater speed and isolation in unit tests. Avdi Grimm originally wrote it and I am the current maintainer.
See my github profile for a complete list.