Glossword is by far the best dictionary tool ever written. It's not the usual "terms database" interface with a search box, it's a full featured dictionary, with important features like translations, abbreviations, definitions, synonyms, linked words, source of information, etc.
It's not limited to ONE dictionary, it was designed to work with "topics", where each topic can have several dictionaries.
Each dictionary can be imported and exported to XML or sqldump, a real timesaver if you already have the data to use on your dictionaries.
Support is top notch, there's plenty documentation, the site is in russian and english, and it's the first application to have a "video" guide, on Windows Media format, for the people who don't want to read README files, and in case of trouble you can contact the author on SourceForge (it's GPL!!!). Excellent pick.