Beware the optical option
Over the years I've had a lot of luck with CDR backups...but that's what it's been: luck.
Estimates of how long a burned CD will typically remain viable vary greatly, but the only safe assumption is that a they're good only for a few years.
I typically still abide by the rule of three (once you have three copies in different places, you're backed-up). A local harddrive is one, a thumb drive is two, and some manner of offsite storage is three. The are some reasonable, free options for the latter...I like Microsoft's SkyDrive. Not appropriate in the case of sensitive, proprietary data (especially if it's not yours), but useful otherwise.