Google Suggest builds up a list of suggestions on the fly, presumably based on previous queries.
Google Scholar -- like citeseer, but better as it indexes journals that are not freely available. The idea I got from Eugene Volokh of the "Volokh Conspiracy" is to enter vulgar sexual terms and get the benefit of the combined academic wisdom on the subject. Hours of fun fun fun!
Integration of image search into regular google search -- when I search for "plutonium", ie http://www.google.com/search?q=plutonium , I get a picture of Bart Simpson holding a glowing green thing. Not useful, but funny.