I use Microsoft's sync toy. It's free, easy to use, etc etc etc.
Download details: SyncToy 2.1
Uses a lot of built in functionality in the windows OS to help it, so it's more resource friendly than your average backup solution.
+1
I need to use it at home. I set it up at the place I used to work in High School, so it backs up all of our machines' documents over the network, and it works like a charm. I set it so Task Scheduler runs each backup job at a different time interval, as well.