GNU/Linux is an UNIX like Operating System, it is the most powerful Open Source Software used on big IT companies. It's the core on most of our devices now adays. I also have knowledge about other OS like: *BSD, OSX/Darwin, Android, Solaris or Windows.
KVM and VMWare are used to virtualize, manage and scale computer systems.
Storage Area Networks based on Fibre Channel are used to storage big data.
Web Servers like Apache, Ngix, Mongrel, or others are used to publish web pages or applications. Most Security Issues dependes on a good configuration of these software.
Tomcat or WebSphere are a good choice to deploy Java applications on your bussiness company.
Web Sites and Design are very important to give a good look to your company. I have knowledge about GIMP, Inkscape, PhotoShop, Illustrator, Corel, ...
Programming Languages like: C#, C++, Korn Shell, PHP, Ruby & Perl. Are very helpful for a system administrador and web developer, I have also knowledge about PHP, RAILS, MySQL, JavaScript, HTML & CSS.
I like to draw with a pencil! But I can choose to use GIMP or Inkscape to make or improve my pictures and Comic Books! ;)
I usually have to use Blender for 3D design, it is Free Software and has a great community and documentation!