Puma is a simple, fast, threaded, and highly concurrent HTTP 1.1 server for Ruby/Rack applications. Puma is intended for use in both development and production environments. It's great for highly concurrent Ruby implementations such as Rubinius and JRuby as well as as providing process worker support to support CRuby well.

A modern web server and application server for Ruby, Python and Node.js, optimized for performance, low memory usage and ease of use.

unicorn is an HTTP server for Rack applications designed to only serve fast clients on low-latency, high-bandwidth connections and take advantage of features in Unix/Unix-like kernels. Slow clients should only be served by placing a reverse proxy capable of fully buffering both the the request and response in between unicorn and slow clients.

A fast HTTP / Websocket Server with built-in Pub/Sub support (with or without Redis), static file support and many other features, optimized for Ruby MRI on Linux / BSD / macOS

A fast, asynchronous, rack-compatible web server.

Ruby-Mongrel2 is a complete Ruby connector for Mongrel2[http://mongrel2.org/]. This library includes configuration-database ORM classes, a Ruby implementation of the 'm2sh' tool, a configuration DSL for generating config databases in pure Ruby, a Control port interface object, and handler classes for creating applications or higher-level frameworks.

A thin and fast web server

Trinidad allows you to run Rails or Rack applications within an embedded Apache Tomcat container. Serves your requests with the elegance of a cat !

A small fast HTTP library and server that runs Rails, Camping, Nitro and Iowa apps.

A fast HTTP server supporting rack.

A Web Server

Hoof is linux variant of pow. It's based on nss, eventmachine and unicorn

Simple, upgradable WebSockets for Thin.

A multi-process web-server written in Ruby 1.9.