Java Java Java!
Every five years or so I take up Java again. In this case, I'm making a Java physics simulation that will run on the Unity server. The clients will be Flash. It's a cool rig because nobody has to work about having the right JRE but me. Everybody has Flash. Unity is cheap as free. I'm psyched about the next year's worth of development.
Java has a steep bump at the beginning of the learning curve. You've got to get your path together, find the libraries then figure out the javac and jar command line interfaces. Then you have the little matter of learning to code Java. The SDK has gotten better and Derek from moock.org is super helpful.
I've gotten over the hump. I have a custom room running on Unity and have established communication with the clients. Now I'm deciding what type of simulation I want to use for the demo app. I have a few things I want to demonstrate:
Synchronous multi-user interaction
Persistent, real-time environment
SCORM gradable results
I'll think of something.