I'll piggyback on your post, because I've been meaning to ask something very similar. Not necessarily a commandline tool, but one that could be described as "TimeSnapper for the Web" (i.e. WebSnapper, if anyone needs a snappy name
Ideally, a program that would take automated screenshots of websites at predefined intervals - be it once a week, or once an hour.
(There are of course very nice Firefox add-ons to take website screenshots, such as Abduction, but AFAIK they can't be automated, and even if they could, that task would effectively hijack the browser as long as the "WebSnapper" would be running.)