Unicorn

Unicorn is an HTTP server for Ruby, similar to Mongrel or Thin. It uses Mongrel’s Ragel HTTP parser but has a dramatically different architecture and philosophy.
In the classic setup you have nginx sending requests to a pool of mongrels using a smart balancer or a simple round robin.
Eventually you want better visibility and reliability out of your load balancing situation, so you throw haproxy into the mix:

http://github.com/blog/517-unicorn