Yomichan is described as 'turns your browser into a tool for building Japanese language literacy by helping you to decipher texts which would be otherwise too difficult tackle. Inspired in part by Rikaichan, it stands apart as an all-encompassing learning tool' and is an app in the Web Browsers category. There are six alternatives to Yomichan for a variety of platforms, including Windows, Online / Web-based, Google Chrome, Firefox and Thunderbird. The best alternative is Rikaichamp, which is both free and Open Source. Other great apps like Yomichan are rikaikun (Free, Open Source), rikaichan (Free), Popup Japanese Dictionary (Paid) and Memento (Free, Open Source).
Japanese-English popup dictionary that lets you look up Japanese word definitions without leaving your current screen! Look up Japanese words simply by copying to the system clipboard, from anywhere, including web browsers, PDFs, ebooks and emails.