In addition to João's answer, I found these open source projects for the RoboCup 3D Soccer Simulation:
- Apollo3D with C++ code on SourceForge (not updated since original release two years ago).
- FIIT RoboCup 3D has multiple languages, with C++ and Java implementations for the robot and a .NET behavior editor on SourceForge (with several commits to the Java code in the last week). Uses Ruby for scripting too. Majority of comments are in Slovak.
- jsshagent with Java code on SourceForge (not updated since first release in Jan 2008).
- robo-cup-soccer3d-framework with Java code on Google Code. Last update Dec 2008. Site in Japanese.
- Zigorat which seems to have two home pages, is another C++ base for robot building, as well as a bunch of other RoboCup 3D tools. Last release was Mar 2009.
I am still yet to find a library for building RoboCup 3D agents in .NET. I've created a base layer for my own C# project that I will release, once I come up with a good name for it!
EDIT I created a new page on the SimSpark wiki to collate various libraries and agents in one place.