I use some shell scripts to back up my web server to Amazon S3. It’s very cheap and off-site, two things that make for a good backup strategy. But what about personal computers? Most people, myself included, simply back up to external hard disk. Off-site backups stored “in the cloud” on remote servers would be a much preferable option. (But most people balk at paying $5-$20 per month for a service like Dropbox, JungleDisk, SpiderOak or Carbonite.)
Arq, by Haystack Software, takes a slightly different approach. It’s designed for those of us who already have Amazon S3 accounts. Arq lets you set a monthly budget, say $5 (which gets you 50GB or storage). The application then keeps the folders you choose mirrored with Amazon S3. It operates on the same hourly schedule as Time Machine, keeping multiple versions of the files. It periodically deletes old versions of the files to stay within the storage space equivalent to the monthly budget you set. It even encrypts your files with a passkey of your choosing.
If you’re selective about the data you back up, the pricing should be fairly reasonable. It seems like a good way to back up data. You could use it in tandem with Dropbox; Arq+S3 for backing up your critical data, Dropbox for sharing and collaborating.