LiveDictionary 1.3b1 now works with OW 5.5! LD allows users to download and manage many different dictionaries. Hitting a hotkey offers definitions immediately in a popup window. Bear in mind that users will need to modify the loader bundle for LD to work with OW. Instructions can be found on the LD forum.
The beta can be downloaded here:
http://forums.eloquentsw.com/viewtopic.php?t=105
The LiveDictionary site is here:
http://www.eloquentsw.com/livedictionary.html
Also note that LiveDictionary can be used in other applications with slight modifications. I have it running in: TextEdit, Safari, NetNewsWire, Adium, iChat, and DevonThink Pro and Devon Agent.