UPDATE: If you are interested in ZFS on linux you have two options at this point:
I have been actively following the zfsonlinux project because once stable and ready it should offer surperior performance due to the extra overhead that would be incurred by using fuse with the zfs-fuse project.
You can see another one of my posts concerning zfsonlinux here.
KQ Infotech has released (currently in closed beta) code that brings ZFS to Linux via a loadable kernel module.
Here is a link to the current and future feature set. The reason that this is exciting is that although other ZFS implementations for Linux have traditionally existed, each of the available options have significant drawbacks. For example ZFS-FUSE is implemented in userspace using FUSE, which has additional overhead due to the context switching that is required while switching back and forth between kernel-space and user -space. .
Another option is ZFS on Linux which provides a stable SPA, DMU and ZVOL layer, but does not however provide a Posix layer (ZPL) that would enable you to actually mount a ZFS filesystem from inside Linux. From what I understand, KQ Infotech has basically taken some of the ZFS on Linux code that was developed by the Lawrence Livermore National Laboratory (LLNL), and actually implemented the missing ZPL layer.
NPR was recently accepted into the closed beta program, and I took some time last week to get this module installed on a Dell Poweredge 2950 running a 64 bit version of Ubuntu 10.04. We are currently testing ZFS under kernel version 2.6.32-24. I have not had a ton of time to test things out, but I would say so far so good. I plan on posting some ZFS and Btrfs benchmarks in the next few weeks after I get some time to better test performance, throughput, etc.