If you've got a knack for developing games and want to create one on an open source multiplayer online framework, then have a look at Arianne. Written in bare C and running on a Java platform, the application lets you focus on creating your game while Arianne does the heavy lifting on its server architecture.
What started out as a tiny app written in pseudo-C++ is now a client-agnostic server framework that uses Python for game description, MySQL in the backend, and a TCP transport channel to communicate with other players.
Arianne's server, Marauroa, is written in Java and is able to run Python scripts to define game rules. "Marauroa is based on a design philosophy we called Action - Perception, notes the development team. "Each turn a perception is send to the clients explaining what they currently perceive. Clients can ask the server to perform any action in their name using actions. Marauroa is totally game agnostic and makes very little assumptions about what are you trying to do, allowing a great freedom to create any game type."
There are a handful of games available on the project's Web site that developers have already created using Arianne (multiplayer Pacman, anyone?). Of course, users are encouraged to put their own newly-created games online as well, so everyone can join in the fun.