What is Vassal? Well Vassal itself is a Java based computer program that allows you to play games in realtime over the web. There are several different programs that do this, but I refer to the genre as VASSAL.

Following are the sites using JAVA realtime game players:
VASSAL	The Father of them all. Has their own server, or you can direct connect to play.
WARGAMEROOM	Has their own Java machine that runs games pretty well