Gambo is a Game Boy emulator written in Java by Omar Rizwan and Vincent Pacelli.
Gambo supports compilation to JavaScript with Google Web Toolkit.
First, copy a base64-encoded ROM to web-app/web/src/main/webapp/game.gb.b64.
Then install Maven and run mvn package in web-app/.
The compiled HTML, CSS, and JS will be in web-app/web/target/web-1/. Run an HTTP server from there and visit index.html to play.