A static site generator. Provides dozens of templating languages (Haml, Sass, Slim, CoffeeScript, and more). Makes minification, compression, cache busting, Yaml data (and more) an easy part of your development cycle.

PayPal Express Checkout API Client for recurring billing.

A monkey patch to speed up Rails' JSON generation time.

Rambulance provides a simple and safe way to dynamically generate error pages.

Add reflection on ActiveModel dynamic attributes that will be created at runtime

This gem provides a graphical frontend for the Taskwarrior task manager. It is based on Sinatra.

Websocket interface on Rack Hijack w/ Rails support

Enables Vagrant to manage virtual machines on a Proxmox server.

Gem for managing multiple analytics services in your rails app.

APIFuzzer gem builds api for finding security issues through a fuzzer

Ruby/JS GUI framework

High method and class churn has been shown to have increased bug and error rates. This gem helps you know what is changing a lot so you can do additional testing, code review, or refactoring to try to tame the volatile code.

A ruby gem for validating credit card numbers

Crummy is a simple and tasty way to add breadcrumbs to your Rails applications.

CSV Validator

Tools to build your double entry financial ledger

Declarative API for specifying features, switchable in declaration, database and cookies.

This library can be used as a module for `fog`.

A configurable and documented Rails view helper for adding gravatars into your Rails application.

Until there is a Kindle API, this will suffice.