BackupURL is a free web service which can create a cache/backup of any URL. This means that the cache created will be available even if the actual website server is down. The process of caching is simple and all you need is to enter the URL you want to cache/back up and click BackupURL. A snapshot of the URL is created and stored.
Once the backup is created the service provides you with the shortened URL to access the cache page. The web page will be stored onto their servers so you can view it even if the original site happens to go down.
Although the service looks good, there is no mention on how long the pages would be stored in the cache on their servers. This means you can use this service for relatively less important URLs.
It looks like DiggMirror. But as you noted, the value depends on how long they are keeping the cache on. Another question that cameup in my mind is: whether it is like waybackmachine, keeping multiple copies of the url over a period of time.