the web archive
(yes instructables is another.)
Ive known about it for a while and wanted to share it.
It lets you go back to past versions of web pages,
simply enter the url and hit "take me back"
For as big as it is, you can search a lot, though not everything is there.
Note: Sadly instructables is not searchable from this site due to a file labeled "robots.txt" which prevents them from searching instructables.