Wiktionary is a multilingual, web-based project to create a free content dictionary of all words in all languages. Developed by Wikimedia Foundation
License: Open Source
Wiktionary is a collaborative, community-driven dictionary that offers extensive coverage of words and phrases with a focus on etymology and multilingual support, while Wordnik provides a rich user-generated content experience with a strong emphasis on example sentences, audio pronunciations, and a user-friendly interface.