A Unity ID allows you to buy and/or subscribe to Unity products and services, shop in the Asset Store and participate in the Unity community.
Separate names with a comma.
More: http://www.infoworld.com/article/07/07/02/mswindows-share_1.html
Anything new on this front?
Today there is e.g. http://www.linuxgamepublishing.com/ or http://www.rune-soft.com/main.php
No Linux market? In the meantime there are more developers who prefer Linux as their development system than Windows according to an Evans Data...