Amazon S3 is among the, or is the lowest cost I've found. Most other cloud storage services limits file size to, say, 2 or 4 GB. So a NAS backup has to split the backup.
My Synology has an S3 client built-in; perhaps so too does QNAP and others.
Amazon has a new service called Glacier. Very low cost per month, but retrieval costs can be high for low-mid volume users. It's intended of mass archiving where required by regulations. Not sure it's viable at the 100GB level.
Big files going out need LOTS of upstream bandwidth/speed. More than most SOHO and residential users have. With this constraint, the best choice is not to use a cloud for offsite. Use USB/eSATA drives physically away or in fireproof safe.
Second to S3, I've found, is CrashPlan - no file size limit.
JungleDisk is an S3 reseller. Good but they like others mark-up S3 about 20%.
CloudBerry is free/shareware front-end to S3.
OpenDrive seems to have put their sins behind them. Small file size limit unless you pay $300/yr or so.
Again, for NAS backups, a push out your modem can be really too slow. Unless your SOHO/biz has a 10-50Mbps upstream (to-cloud). That's very $$$.
Last edited by stevech; 12-03-2012 at 10:03 PM.