If your focus is primarily on System Administration, Perl gives you the greatest flexibility of any cross-platform language I know. However, despite my personal distaste for .NET, it is an excellent platform to get used to if you work ever need Windows-specific applications (esp. if you need to do advanced work with Active Directory).

If .NET doesn't jingle for you, I recommend Java. Again, I personally dislike the language; but, it is a cross-platform language that has free (and Free) compilers and runtime environments. It's used in many places for automation and security work, so it's a good skill on a resume, and it's not far off from MS's C#.NET if you end up needing to go that direction.

If you have solid Java (esp. WebSphere and Tomcat) experience and solid Perl skills, you'll never have trouble finding a job.

However, if you simply want to challenge yourself, stretch your brain, and improve your overall programming ability, I suggest LISP. It seems like most of the popular languages are becoming more like LISP on a daily basis, anyway.