Last weekend ZFS On Linux 0.6.5.4 was released. ZOL v0.6.5.4 brought support for the Linux 4.4 kernel while continuing to support older kernel versions going all the way back to Linux 2.6.32. This ZFS On Linux update also brought a number of stability fixes, better support/stability for NFS-exported snapshots, and a variety of other fixes.
ZFS On Linux support still is left outside of the mainline Linux kernel due to license differences with the dependent code that would need to be mainlined. At least, however, we are seeing more Linux distributions offer their own packages or enabled kernels with support for this advanced file-system. With Ubuntu 16.04 there will also be out-of-the-box ZFS support.
You can download this latest ZFS On Linux release via GitHub.
Meanwhile, released yesterday was an updated patch of the Reiser4 kernel support. From SourceForge you can download the patch to apply against the Linux 4.4.0 stable kernel for enabling support for this file-system. While there aren't any licensing woes with Reiser4, there's been no talk recently about trying to mainline support for this file-system since there are limited developers left working on the project and no backing by any of major companies having a stake in Linux. Most of the Reiser4 work in the past few years has been maintenance with just providing fixes, support for new Linux kernel releases, and few new features.
Who's interested in a fresh Linux 4.4 file-system comparison of the mainline contenders (F2FS, Btrfs, XFS, EXT4), compared to ZFS and Reiser4? Let me know in the forums if you are interested.