http://www.archive.org/
This is the one I use to locate archived websites.
If it doesn't show up, it means that the site had a file on their website that prevented the archive from trolling it for the archive.
When I had my site up, I didn't want it to be archived, so I put such a file on it to prevent it from happening.
If you know the website address, just enter it into the address bar and if it comes up as not found, you'll know it isn't online.