DeltaCopy is an rsync wrapper for windows. Have not used it myself yet, but rsync is a standard method of syncing files in the unix world. It is capable of syncing entire hard drives, easily recovers from connection issues because it can pick up where it left off in a sync. It can either compare and copy entire files, or send only the bitwise differences between files. You can also tunnel rsync over ssh to use securely over the internet, or use it between network shares, or between local hard drives or directories.
My usage would be along the lines of:
rsync -avz dirA dirB
rsync -avz dirB dirA
where "-avz" specifies to copy in 'archive' mode (recursive, etc), v=verbose, z=use compression for the transfer (useful if over a network) and the directories are source and destination respectively. Swapping on the second command gives you a two-way sync.
Another unix like tool that I believe has a windows version is Unison which is specifically built to handle 2-way and 3-way syncs, and it also has conflict resolution options in case the same file changed in multiple places.
The final option which I use for lightweight backups on windows is SyncToy, a free download from Microsoft that allows you to define folder pairs and preform either one or two way syncs. It is directory based and does not have the network copy functions of the previous two programs.
All are free.