The cost for these services splits neatly into two piles: flat-rate storage and per-gigabyte (metered) storage. Of those tested, only Jungle Disk and SpiderOak are metered.

Flat-rate services charge a per-computer rate paired with unlimited storage, while metered services allow unlimited computers to have pooled data. iDrive is an exception--it offers 150GB storage for up to five computers for a flat monthly rate.

The flat-rate providers typically charge about $5 per computer on a monthly basis, although several only allow advance payment for a year or more at a time, or offer a discounted yearly option.

CrashPlan offers a $100-a-year rate for an unlimited number of computers storing an unlimited quantity of data. This is less expensive than all its flat-rate competitors for two computers, and vastly cheaper for more than two.