Google Scholar. It doesn’t find everything – hardly surprising – but it did find interesting things I didn’t previously know about, on topics where I thought I knew the literature. Count me as impressed, and likely to use it further.
Update: Well. Having just replied to a referee report, I can say that “Google Scholar” was exceedingly useful. Now it just needs to be linked to amazon.com’s “look inside a book”, wikipedia, MathWorld, and PlanetMath.
It also appears to be a great way of constructing reading lists: search for your topic of interest, and look to see what comes up first. Eventually, pagerank may be a better measure of impact and utility than citations. Describing potential problems caused by this is left as an exercise for the reader…