Glosbe - Huge collaborative massively multilingual dictionary.
StoryMill - Developers and publishers of Mac, Win, and iOS productivity, home and office and writing software such as MacGourmet, Paperless and many more. Based in MN.
Visuwords - The fun and free way to look up definitions. THE graphical dictionary and visual thesaurus.
Manuskript - Open-source tool for writers.
Wordnik - the biggest online English dictionary, by number of words
Novelize - Novelize is an online novel writing app designed to keep authors on track.