There are a lot of good options out there, but you may want to take a look at two free programs first...ViceVersa and Easy2Sync. If you're looking for something more advanced write back, but either of those should get the job done.
Hope this helps,
I have a new 160GB Dell desktop and would like to keep the files from it synchronized with my 80GB notebook. How do I do this?
I also own an external 160GB USB connected hard drive. I have PC Relocator and have been looking at SyncToy from Microsoft.