Set the Wayback machine
I've been hearing about this, or variants on it, for a while, but it's only just been made available to the public: the Internet Archive, in conjunction with Alexa, has just released the Wayback Machine, "over a hundred terabytes of old Web pages, stored on 300 PCs in a basement in the Presidio." In theory, you can use it to look up old Web pages dating back to 1996, as described in this Infoworld article; in practice, traffic is extraordinarily heavy even in the middle of the night, and you may not be able to get any useful info from the system 'til they install some new servers in a few weeks.
Note: if there's anything you don't want archived on your site, probably best to use a robots.txt file, as described in the FAQ.