I really love the 'define:' function; I used it all the time before Mac OS added the dictionary widget, and I still use it if I want to look up something that's not likely to be in a dictionary (like a technical standard). It's great!
I use the site: operator a lot. It's very helpful for searching sites when their own search functions suck. I also use google as my calculator, since I pretty much always already have a browser window open already. Ditto for define: as a dictionary.