Glosbe - Huge collaborative massively multilingual dictionary.
Clever - syncing between education applications for K-12 schools
Visuwords - The fun and free way to look up definitions. THE graphical dictionary and visual thesaurus.
PowerSchool - PowerSchool provides a K-12 education technology platform for operations, classroom, student growth, and family engagement.
Wordnik - the biggest online English dictionary, by number of words
Claroline - Claroline is a collaborative eLearning and eWorking platform.