I just did a review of PogoPlug, an interesting cheap Cloud Backup Service that allows you to backup for only $4.95 a month, so that's like $29 a year. I think that's fantastic, considering Dropbox offer $50 for 500Gb, so with this, not only you can backup your computer online but your entire family's.
They have the software you need to connect with your Macbook, PC or iPhone/Android to your backed up files, which is fantastic. And more importantly, you will have the sense of security knowing your backups are online, so if your house burns down (and I hope it doesn't), you don't have to rely on physical backups that may be compromised.
So to get things started, you can click here and get 5GB for free, so you can test it out. If you like it, then do the $29 a year thing. I will certainly try it and see how it goes for a whole year. Certainly cheaper than even buying a hard drive, even at today's prices.