You can never have too many dictionaries and other writing tools.
One of my favourites has been Agile Tortoise’s Terminology, which only runs on iOS. It uses an offline version of WordNet, a combination of dictionary and thesaurus created at Princeton University. As this is based on American English, non-US users will find that usage and spelling vary from British, Australian, and others.
Greg Pierce at Agile Tortoise has now released a free OS X dictionary based on WordNet, which effectively brings Terminology to your Mac.
Download the Zipped archive from here, unZip it, and move the uncompressed dictionary file into /Library/Dictionaries. You will have to authenticate to do that. Once it is there, open the Dictionary app and, if it does not appear in the list of dictionaries on offer, enable it in Dictionary’s Preferences.
Thanks to Brett Terpstra for drawing attention to this, to Agile Tortoise for providing it, and to the folk who built and maintain WordNet.