OpenWarfare is a multi-purpose open source mod for the Call of Duty series. The project started on May 7th, 2008 and quickly grew to become one of the leading mods for the Call of Duty series. The mod supports new gameplay features to make the game more fun as well as league competition features like ready-up, strategy time, checksums, variable changes detection, rulesets, etc. Features in the OpenWarfare mod are not forced at all and can be enabled/disabled based on what the server owners like.
Read more at http://openwarfaremod.com