dict66 is described as 'Easy to use online dictionary and multilingual translation service' and is a Dictionary in the Education & Reference category. There are more than 10 alternatives to dict66, not only websites but also apps for a variety of platforms, including Android, iPhone, iPad and Chrome OS. Most alternatives are Dictionaries and Translator Apps. The best alternative is GoldenDict, which is both free and Open Source. Other great sites and apps similar to dict66 are Wiktionary (Free, Open Source), WordReference (Freemium), Merriam-Webster (Free) and Dictionary.com (Free).
GoldenDict is an open-source dictionary program for PC computers that gives translations of words and phrases for different languages. It allows use of several popular dictionary file formats simultaneously.
Google's dictionary extension on Google Chrome. With it, you can double-click any word to view its definition in a small pop-up bubble, or search the complete definition of any word or phrase using the address bar dictionary.
Online Dictionary - Multiple dictionaries including: English dictionary, medical dictionary, legal dictionary, financial dictionary, computer dictionary, thesaurus, dictionary of acronyms and abbreviations, dictionary of idioms, thesaurus, Columbia encyclopedia, Wikipedia...
Lion allows you to send a query to the German-English, -French, -Spanish and -Italian LEO dictionaries (Chinese is currently not supported) with a click of your mouse from almost all MS Windows applications.