Numerous Middleware components 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::Heartbeat Add a configurable heartbeat/health-check/ping url to your app
• Rack::Honeypot acts as a spam trap.
• Rack::GoogleAnalytics embeds Google Analytics tracking code.
• Rack::Embed embeds small images via the data-url (base64) if the browser supports it. This reduces http traffic.
• Warden General Rack Authentication Framework
• 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 implements Linked Data content negotiation.
• SimpleRouter
• 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.
• Rack::Batik::SVG transcodes SVG pictures to JPEG
• Rack::Referrals extracts referring search engine info
• Muster parses some or all of the query string in varying formats into hashes. Helps adding human friendly (not rack/rails nested) query string options to APIs and such.
• Moneta adds Rack::MonetaStore (places key/value store in env, support for per-request caching), Rack::MonetaCookies (allow Moneta to be used to store cookies) and Rack::MonetaRest (expose a key/value store via HTTP/REST)
• RouteDowncaser makes all routing in Rails case-insensitive.
• Rack::Attack A DSL for blocking & throttling abusive clients
• ChromeLogger A Ruby library that implements the Chrome Logger spec as Rack middleware
• Rack::DetectTor Detect Tor exit users
• Rack::RackTorBlock Block access to a rack application from any client accessing from the Tor network
• Rack::Turnout Easily put your Rack apps into maintenance mode
• Rack::CAS Simple CAS client authentication
• Rack::AcornCache Configurable HTTP proxy caching solution
• Rack::Fraction Middleware which lets you to execute arbitrary code just for a certain fraction of requests