Active software-based file replication?

[kmoogle]

I was curious if anyone could recommend any software-based replication
software for a RH-based fileserver. I currently have 1 TB NFS server
hosting files for 10 linux clients and would like to add a second 1TB
fileserver - but have the filesystem actively replicated and available
should the primary server fail. I was looking for a solution similar
to Constantdata.com's Constant Replicator - but would appreciate any
suggestions.

Thank you for your assistance.