Missing in action for many months after a server hard drive failure, the webkit2png utility by Paul Hammond reappeared in August. It uses WebKit to automatically render PNG images of web pages. It beats regular screen grabs mainly in its ability to render full-length images – as if you had an infinitely tall monitor. By default it produces three versions: an actual-size “clipped” version, an actual-size long version, and a thumbnail-size long version (here’s an example). It requires that you have PyObjC.
There’s also a newer utility called Paparazzi that puts a nice GUI interface on the concept (though it is not in fact based on the webkit2png code).