Zapper [Windows] looks interesting: you highlights words and it does context sensitive searches for you, and performs little services (looking up books, translating into Spanish were two I saw on the tour). You can even create your own searches, in a primitive kind of way.
Not something I'd use. I'm fairly happy finding stuff on the www, and only really happy using new technologies when I can subvert them to my own desires. Where's the backend exposure? Where're the xml hooks? But then I guess I'm only a small market.