It's also useful to note that you can instruct Google to not cache your site or individual pages/files on your site. You just need to add <META NAME=”ROBOTS” CONTENT=”NOARCHIVE” /> to the header of the pages on your site that you don't want indexed. Sometimes if you have time sensitive information, or need to remove something from your site for legal reasons, it's helpful to keep Google from caching your pages so someone can't look up an older copy of them.
The Wayback Machine is similar, but I've noticed that it ignores stuff like javacript and flash when creating archives of your sites.