I was looking into this some time back.. The best I found was JOGRE (http://jogre.sourceforge.net/
) and Ive been using this
to develop some online games (including Chess!)... The code is pretty well document with loads of examples. The Server has a nice little gui which is useful for monitoring client/server communication.
Not sure if its used on any websites yet (and am quite far off from finishing).. There are quite a few other solutions but havent investigated these yet. Let me know if you find something better.