I've been trying Box.net out for about a month - pretty nice, and they offer a free plan and also a couple of inexpensive plans also. I also tried Live Mesh but that is still a mess currently.
Otherwise, backup to external drives and optical media, though those will eventually not be usable - hopefully it won't matter by then! (I ain't getting any younger; maybe some media will outlast me!)
PS - Monitoring the S.M.A.R.T. status is a good thing, but I can say from experience that drives can - and do! - fail even with a perfectly good S.M.A.R.T. rating! Be careful even when it looks good!