Data sources

is built using a number of free and open-licensed data sources, which it could not exist without.

These sources are as follows:

OPTED
A public domain English word list dictionary, based on the public domain portion of "The Project Gutenberg E-text of Webster's Unabridged Dictionary" which is in turn based on the 1913 US Webster's Unabridged Dictionary. (See Project Gutenburg)
Wiktionary
WordToys does not use Wiktionary itself directly as a source. Rather, Wiktionary has a fantastic list of links to other resources where you can grab public domain dictionaries and thesauruses.
WordNet®
WordNet® is a large lexical database of English. Nouns, verbs, adjectives and adverbs are grouped into sets of cognitive synonyms (synsets), each expressing a distinct concept. WordNet® is a registered tradename of Princeton University.