Hammer is a fine-grained snapshot filesystem designed for large disks/media. Its sweetspot begins at 500GB, and it can handle up to 1 Exabyte of data. Hammer runs a snapshot every every time the system syncs its mounts, in effect every 30-60 seconds. This history is retained and can easily be sifted through.
Hammer mounts instantly (no fsck needed on crash recovery) and offers queueless mirroring. It also has no inode limitations; after a previous experience of having to recreate an entire filesystem to get over the inode issues caused by a user with millions of very small files, this is enough by itself to interest me!
More information available here.
Hammer is currently in beta, so it's not a good idea for production systems yet, but it's expected to be production-ready later this year. If I were building a cluster I'd be tempted to experiment with both Dragonfly and Hammer.