Operating Systems
Red Hat Enterprise Linux
Long regarded as the definitive enterprise Linux distribution, Red Hat provides a robust, fully supported system ready for enterprise workloads.
http://www.redhat.com/rhel
SuSE Linux
SuSE Linux is the operating system for all of Novell's offerings. Suitable for all enterprise workloads (directory services, databases, web servers,...) SuSE Linux and its OES offspring come with the backing of this industry heavyweight.
http://www.novell.com/linux/
Solaris
Solaris has long been the leader in commercial Unix. The addition of the award-winning ZFS storage technology extends Solaris' lead in the commercial Unix environment.
http://www.sun.com/software/solaris/
AIX
AIX provides the level of functionality, strength and reliability the industry has come to expect of IBM's products. This commercial Unix continues to provide a platform suitable for any application.
http://www.ibm.com/aix
HP-UX
Running on both PA-RISC and Itanium servers HP-UX is a robust, feature rich Unix platform.
http://www.hp.com/products1/unix/