monit is a free, open source process supervision tool for Unix and Linux. With monit, system status can be viewed directly from the command line, or via the native HTTP(S) web server. Monit rose to popularity with Ruby on Rails and the Mongrel web server, because a tool was needed that could manage the many identical Mongrel processes that needed to be run to support a scalable Ruby on Rails site, and monit was fairly uniquely suited for the needs of the Ruby on Rails community.
Monit is licensed under the terms of the Affero General Public License (AGPL) and is written in C.
Many popular Rails sites have used monit, including Twitter and scribd.