There are two that I know of: Carbonite and Mozy. I use Carbonite and I
like it. For less than $5/month, it's one very important responsibility
that I no longer have to worry about.
They have a free trial period. Try it and see what you think. Everything
the literature says is true. You just ignore it and let it do its thing
quietly in the background, with no limit on storage space. (My daughter
trusts them with 13GB of photos.) There are intelligent exceptions to what
they'll back up automatically (e.g., .exe files -- there are a LOT of
those), but you can override those exceptions file by file if you want. You
have full control, including the ability to lower its priority or pause it
entirely (for a specified period of time), if you think it's bogging you
down at all. e.g., when they occasionally "scan the disk for files to back
up" and I would rather grant complete hard drive access to other activity.
Generally I have the feeling that they're "on top of it" as soon as you have
created or modified a file in any way, and versioned backups are kept for a
month, just in case you make a mistake. I've just hit on the highlights
here. If you let me recommend you as a new customer, they'll extend my
contract.