| Z/OS | IBM | 2001 | — |
| Z/VM | IBM | — | — |
| Z9001-OS | VEB Robotron-Meßelektronik „Otto Schön“ Dresden | — | — |
| Zapple Monitor | Princeton | — | — |
| ZDOS | — | — | — |
| Zentyal | — | — | GNU General Public License |
| Zenwalk | — | — | GNU General Public License |
| Zephix | — | — | — |
| Zephyr | Wind River Systems, Linux Foundation | — | Apache Software License 2.0 |
| Zeroshell | — | — | GNU General Public License |
| Zeta | YellowTAB | — | — |
| ZevenOS | — | — | GNU General Public License |
| Zex/OS | — | — | — |
| ZFSGuru | — | — | — |
| Zorin OS | Zorin Technology Group Ltd. | 2009 | GNU General Public License |
| ZRouter | — | — | — |
| ZXVGS | — | — | — |
| ZyNOS | Zyxel | — | — |