All-Knowing Mind is a Java/SWT multi-language, OS-independent game server browser focusing on first person shooters similar to Yahoo's now defunct All-Seeing Eye.
quake4j is a Java implementation of the original Quake engine. The game client can be launched from any web browser with one click, and lets you play deathmatch games with other quake4j users.
Ruin is a 2.5D Java game world that can run from a browser. It's like a 3D MUD. The game looks similar to Doom or Heretic. View the home page for a demo (http://ruinworld.sourceforge.net).