Yomichan от FooSoft

Yomichan turns your browser into a tool for building Japanese language literacy by helping you to decipher texts which would be otherwise too difficult tackle. This extension was inspired in part by Rikaichan and Rikaikun, but it stands apart in its goal of being a all-encompassing learning tool as opposed to a mere browser-based dictionary, while adding advanced features such as EPWING support.