ReadNihon is described as 'Generate website Furigana by your Japanese level'. There are four alternatives to ReadNihon, not only websites but also apps for a variety of platforms, including Windows, Firefox, Google Chrome, Linux and Mac. The best alternative is Yomichan, which is both free and Open Source. Other great sites and apps similar to ReadNihon are rikaikun (Free, Open Source), rikaichan (Free) and Popup Japanese Dictionary (Paid).
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.
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.