I've used Dave's Quick Search Deskbar for quite some time, but never really explored all of it functions. I use it mainly for google and reverse telephone number lookups.
Some of the uses you listed I have known of, some I didn't. I do use the reference portion (Language and General) quite alot also.
[Edit] There is a new version of Dave'sout now. Version 3.1.8 (January 18, 2004)