Mainframe / Server operating systems (multi-user)
Split between UNIX/Linux and Microsoft.
Been that way for a long time.

Usage share of server operating systems, 2008.
Stats from
IDC Server Tracker.
Pie chart generated with
piecolor.com.
- Update:
End 2010.
Server OS share (by percentage of revenue):
- Windows 42.1
- Unix 25.6
- Linux 17.0
- z/OS 11.3
- Other 4.0
Server operating systems