For historian of 18th and 19th century America, Google Book Search is the most radical expansion of available research resources since--I don't know, since JSTOR. (Because of copyright it is much less useful to 20th century historians). Suddenly the number of period books at my disposal has gone from the few thousand at my library to a few hundred thousand--all of them downloadable and subject to keyword searches.
(If you use Google Books as much as I do, it pays to keep Inside Google Book Search, the official blog, in your RSS feed.)
