Andrew File System (AFS) is a distributed, network filesystem built for high performance. OpenAFS is an open source branch of the original AFS now owned by IBM.
Gluster is a distributed NAS filesystem now maintained by Red Hat. Scalable network filesystem that uses common off-the-shelf hardware.
Global File System (GFS2) is a shared-disk or clustering filesystem. Multiple nodes can share the same disks. All nodes are peers. Maintained and supported by Red Hat.
Distributed Replicated Block Device (DRBD) is a distributed replicated filesystem build for HA clusters. It is maintained by LINBIT.