Distributions

The following open-source operating system distributions use Open ZFS (listed alphabetically):

OmniOS

 * general-purpose server OS
 * download page; source code
 * commercial support available from OmniTI

OpenIndiana

 * general-purpose server OS
 * download page; source code

FreeBSD

 * general-purpose server OS
 * download page; source code

FreeNAS
incomplete list, please help!
 * NAS appliance software, based on FreeBSD
 * commercial support available for sister product TrueNAS from ixSystems

Debian GNU/kFreeBSD

 * Debian variant with ~90% of the same software packages, based on GNU libc, but using the FreeBSD kernel which provides ZFS
 * Installer supports creation of ZFS pools, installing to them, and booting directly from them with GRUB2
 * We'd like to know how we're doing, and what major features are missing (DTrace comes to mind)

ZFS on Linux

 * ZFS on Linux provides self-building packages for Debian, Fedora, RHEL/CentOS/SL, Ubuntu and build instructions for several other distributions

Gentoo

 * Gentoo provides first-party ZFSOnLinux packages to itself and its derivatives.