A search engine that lets you search by parts of speech, so that you can use the internet as your corpus and do all sorts of cool linguisticky things.
http://lse.umiacs.umd.edu:8080/ Sure would have been useful for DBP and Brady when I was interning for them in the summer of '99. I don't know that I'd ever need to use it, but it definitely brings out my inner symsys major.