Free? I'm not aware of any. Cheap? The cheapest I know of is JungleDisk [ www.jungledisk.com ] - they've included a workgroup version and now support Windows, Mac and Linux operating systems. I don't know that its the easiest to set up, but at $0.15 per GB per month - plus some very small transfer fees and licensing fees. It's pretty cheap.
The other two that I know of are Mozy and Carbonite. They are, in my opinion, neck-and-neck competitors when it comes to the service they offer. I prefer Mozy, but either would work for you. They each have "home" accounts for sale at about $50 / yr with unlimited storage. Mozy also has a MozyPro service, which is more managable from a control panel, but its a bit of a bear to use.