JavaGear is a Sega Master System (SMS) and Game Gear (GG) Emulator for Java. It may runs as either an applet, embedded in a web page, or as a stand-alone application.
JavaGear was originally conceived as Chris White's final year university project. The full report can be found on his JavaGear website.
This is the site for the JavaGear project on SourceForge, but it is not the only home of JavaGear. Chris White's official JavaGear site (www.javagear.co.uk) contains a lot of useful information as well as a version of JavaGear for mobile devices, called JavaGear ME.