ZFS

Fast file system creation: The creation and startup of additional zones (“SmartMachines” in Joyent terminology) – in other words, adding new paying customers — is nearly instantaneous.
Data integrity is guaranteed, with particular emphasis on preventing silent data corruption.
Storage pools: “virtualized storage” makes administrative tasks and scaling far easier. To expand storage capacity, all you need to do is add new disks (hard disks, flash memory, and whatever may come along in the future) to a zpool.
Snapshots: ZFS’ copy-on-write transactional model makes it possible to capture a snapshot of an entire file system at any time, storing only the differences between that and the working file system as it continues to change. This creates a backup point that the administrator can easily roll back to.
Clones: Snapshots of volumes and filesystems can be cloned, creating an identical copy. Cloning is nearly instantaneous and initially consumes no additional disk space. This facilitates the rapid creation of new, nearly identical, VMs.
The ARC (Adaptive Replacement Cache) improves file system and disk performance, driving down overall system latency.

News

The Future of LibZFS illumos user group meeting Jan 2012
ZFS Feature Flags illumos user group meeting Jan 2012
ZFS: The Last Word in File Systems - 3-hour deep dive video with Bonwick and Moore
- various versions of the slides are available, including this from someone who hated Sun and yet thought ZFS "Worth running Solaris to get". And another version.
ZFS on Solaris Internals site
ZFS in the Trenches
Becoming a ZFS Ninja
ZFS Revisited - Understanding ZFS & ZFS ARC/L2ARC (blog post)
Activity of the ZFS ARC (blog post)
ZFS Workshop at LISA 08 (6 hours of video!)
all Deirdré's ZFS videos