Darkstar

Project Darkstar, or Sun Game Server as it is also known, is a now open sourced massive multiplayer game framework implemented by Sun.

It supports persistent objects on the server that can execute tasks (in parallel on one machine, or on different server computers). It hides all the messy details from the game programmer, presenting a very clean and simple API to the programmer.

There is one server tutorial and one client tutorial, they are only available inside the Darkstar downloads currently. The Project Darkstar homepage only has a presentation from JavaOne in its documentation section.