My data files are on a 1TB RAID-5 array in my main home machine, and I drop an occasional mirror image onto an external USB drive. (Not real consistent about copying it over, since it's real slow.) I recently saw somewhere (I think it was Newegg.com) advertising 1TB SATA drives for $99. I plan to keep my eyes on the prices, and before long I figure I'll set up an external SATA drive instead, so it'll be faster to back up. At that point I'll probably actually start running backup software to automate the process. (Any suggestions?) I also drop occasional copies of things like my My Documents folder onto my other machine, which has about 1TB of non-RAID storage.
My apps and OS are on a separate physical drive, and I keep periodic restorable images of it in a few places.
Carbonite sounds like it might be a good idea, just to have some off-site backup as well. I plan to look into it more.
I'm still amazed at how the prices for disk and flash storage have dropped over the years. I recently picked up a Corsair 8GB USB flash drive for $10 (after the $10 rebate). That's roughly 200 times more storage than my first laptop had, and about $3,990 cheaper. When I first go into the software business in 1991, a terabyte of disk storage (which was unfathomable at the time) would have cost about $325 million. Now you can buy it for $100 if you shop around.