File Sync between two separate servers

Hi All, We have a situation where we have a primary file server, and a back up one, what I would like to do is sync up changes from the primary file server (directory /somestuff for example) with the backup server's /somestuff directory. Ala, any files that have been changed on the primary get copied to the backup machine say once a day? I know mirroring HD's in the same system would be a better way to go, but has someone done something like this before? -Bill