[tianshuo]

As a visual dictionary, this is not as useful as conceptnet, compare vs "conceptnet" on common words such as "red", "apple".

[phaerus_iconix]

Thanks for pointing out conceptnet. On GraphWords, "synonym" only returns "equivalent word", conceptnet adds "antonym" (as entire category!) as I'd been looking for...

[rhdunn]

WordNet has antonym and other semantic relationship information, so it should be possible to add that to GraphWords.