Numerous Middleware exist in the Rack library itself, as well as the official Rack contributions project.

• JSON-P
• Rack::Cache is suitable as a quick drop-in component to enable HTTP caching.
• Rack::ESI is a small (but still very useful!) subset of ESI (Edge Side Includes).
• CloudKit includes OpenIDFilter, OAuthFilter, and RESTful JSON storage
• Rack::Config – Shared configuration for cooperative middleware.
• Rack::OpenID provides a more HTTPish API around the ruby-openid library.
• Rack::Debug
• Rack::AbstractFormat
• Rack::RespondTo allows triggering different actions based on requested media type.
• Rack::SupportedMediaTypes specify an app’s supported media types.

• Rack::AcceptMediaTypes determine the request’s prefered media type.
• Rack::MultipartRelated parses multipart/related requests and rebuild a simple/merged parameters hash.
• Rack::Honeypot acts as a spam trap.
• Rack::GoogleAnalytics embeds Google Analytics tracking code.
• Rack::ChromeFrame brings HTML5 and other open web technologies to Internet Explorer.
• Rack::Embed embeds small images via the data-url (base64) if the browser supports it. This reduces http traffic.
• Warden
• Rack::StaticFallback bounces or redirects requests to missing static files.
• Rack::Throttle provides logic for rate-limiting incoming HTTP requests to Rack applications.
• Rack::LinkedData
• SimpleRouter
• Rackamole Observe your web applications in the wild!
• Cylon Disallows application indexation by search engines except in production. Rack Middleware and Rails Engine.
• Slogger::Rack::RequestLogger if you want to log on Syslog.

see also Coderack Middlewares