Mongrel is a web application server for Ruby applications. Although it was very popular in the early days of Rails hosting it effectively stagnated and was dropped by the original maintainer; it has since been somewhat eclipsed by alternatives like Unicorn (itself a derivative of Mongrel) and Passenger.

I personally used Mongrel to host this site from its initial deployment (in March 2008) up until I moved to the cloud and switched to Unicorn (in February 2010).

Official site