The Free Dictionary comes very close to beating out dictionary.com. For one thing, there aren’t any pop-ups. But the real geeky advantage they have is the contextual examples from classic literature. For almost every word, you’ll get at least three quotes from Wilkie Collins, O. Henry or Mark Twain, and you’ll be able to click on the precise place they appear. There are some minor problems with this approach. Davenport, for example, seems to bring up character names and the city rather than the sofa. But short of paying big bucks for The OED Online, this isn’t too bad of a substitute.