Popup Japanese Dictionary is described as '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'. There are four alternatives to Popup Japanese Dictionary for a variety of platforms, including Windows, Online / Web-based, Firefox, Google Chrome and Linux. The best alternative is Yomichan, which is both free and Open Source. Other great apps like Popup Japanese Dictionary are rikaikun (Free, Open Source), rikaichan (Free) and ReadNihon (Free).
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. Inspired in part by Rikaichan, it stands apart as an all-encompassing learning tool.