One of the more unusual massively multiplayer games currently running on the internet is SimCorps, which uses the results from matches played in a whole range of different games to effect the situation in a persistent online world. There are currently two campaigns running - one real world campaign based around a fictional invasion of the Panama Canal, and one science fiction campaign set in deep space over a thousand years in the future.
Custom missions are available to download for many of the games, and there is a wide range on offer. The real world campaign supports several "Jane's" titles (their "688i" submarine sim, "F/A-18" flight sim, "USAF", and "Fleet Command"), as well as tactical combat game "Rogue Spear". The futuristic campaign supports "Freespace 2", "Battlezone II", "Homeworld", "Alpha Centauri", and "Unreal Tournament". Something for almost everyone then. Basic membership is free, so to find out more head over to the SimCorps website.