If you go on Monster.com or something comparable you may see jobs that are requesting you to have various certifications. Most certification requests are for Microsoft, but there are various server jobs that require substantial Linux expertise. If you are going to be managing a Unix server knowing how to use Linux will help alot. You can grab a free Linux distribution that will give you access to basically the same environment as Unix. IBM has AIX which is a Unix derivative; but your not going to get your hands on AIX from a free download on the Internet. The same goes for Open SCO.
Learning Linux is the next best thing. To show employers that you have Linux experience you want certifications from the Linux Professional Institute. LPI provides a standardized test that is representative of an excellent understanding of Linux environments, and an ability to manage them.