Thin: A Ruby HTTP Daemon That’s Faster Than Mongrel
Thin is a new Web server / daemon written in Ruby by Marc-André Cournoyer that uses the EventMachine, Rack, and Mongrel libraries. EventMachine makes it super fast at processing network I/O, Rack makes it easy to integrate with existing Ruby Web app frameworks, and Mongrel helps it parse HTTP. So, yes, the title is slightly misleading. Thin actually relies on Mongrel, but is ultimately faster than it, even against Mongrel's EventMachine-enhanced guise.
You can get started with Thin with a simple sudo gem install thin and then you can use it with any Rack supporting Web app / framework. With a Rails app, for example, a simple thin start in the base directory will get things moving.