Uptimes project
http://uptimes-project.org/hosts/os_breakdown
One among several interesting statistics:
,----[ Quote ]
| Operating System Average CPU Load
| BeOS 15%
| Darwin 65%
| Embedded 99%
| FreeBSD 2%
| Linux 2%
| mIRC 0%
| OpenBSD 3%
| OpenVMS 17%
| OpenVMSClust 5%
| OS/2 14%
| Unknown 0%
| Windows 35%
`----
Linux is favoured by many criteria.