Fresh off the release of an updated version of OmniGraffle, The Omni Group has released a new version of their network dictionary application, OmniDictionary. OmniDictionary allows users to easily log into a network dictionary server to easily check word spelling and meaning. According to The Omni Group:
OmniDictionary is a Mac OS X client for network dictionary servers. Itis pretty straightforward - type a word into the entry field, hit return, and the definition is displayed in the definition area. If the dictionary server supports multiple dictionaries, you can select your desired dictionary from the dictionary popup to limit queries to that specific dictionary, or you can query all the dictionaries. OmniDictionary also provides a "Define in OmniDictionary" Service, so you can select words in your other applications and hit cmd-= to see the definition. (Note that, at least by default, Mac OS X does not register services from applications on the network - OmniDictionary must be installed in your machineis Applications folder - or a subfolder of that - for the service to be registered.)
Since OmniDictionary is simply a client for dictionary servers on the web, you have to be connected to the net (or have access to a local dictionary server) in order to be able to perform lookups. Check http://www.dict.org/ for more information on these dictionary servers.
OmniDictionary is available for free. You can find more information at The Omni Group web site.