XFS is a journaling filesystem w/ rapid recovery from system crashes and the ability to support extremely large disk farms.

XFS is licensed under the GPL. w00t!!!

XFS provides 32-bit and full 64-bit file capabilities and can handle filesystems up to 1 terabyte in size. The XFS file system integrates volume management, guaranteed rate I/O, and journaling technology for fast, reliable recovery.

File systems can be backed up while still in use!

If you are unfamiliar with journaling file systems- simply put- a journaling file system keeps a 'journal' of what it is writing at all times. In the event of a system crash the filesystem need only refer to the journal to know what was and what was not written back to the disk properly. Previously fsck had to check the ENTIRE disk after a crash- not anymore. The quick recovery time of a journaled file system is attributed to this fact. ReiserFS, JFS, XFS and ext3 are all journaled file systems.