Someone recently asked me what language a sysadmin should learn.
If you are a sysadmin for Windows the answer is pretty easy: PowerShell.
The answer is more complicated for Unix/Linux sysadmins because there are more choices. Rather than start a "language war", let me say this:
I think every Unix/Linux sysadmin should know shell (sh or bash) plus one of Perl, Ruby, Python. It doesn't matter which.
The above statement is more important to me than whether I think Perl, Python or Ruby is better, or has more job openings, or whatever criteria you use. Let me explain:
It is really important to learn bash because it is so fundamental to so many parts of your job. Whether it is debugging an /etc/init.d script or writing a little wrapper. Every Unix/linux sysadmin should know: how to do a for loop, while loop, if with [[ or [, and $1, $2, $3... $* and $@, case statements, understand how variable substitution works, and how to process simple command-line flags. With those basic things you can go very far. I'm surprised at how many people I meet with a lot of Unix/Linux years under their belt that can't do a loop in bash; when they learn how they kick themselves for not learning earlier.
The choice of perl/python/ruby is usually driven by what is already in use at your shop. Ruby and Python became popular more recently than Perl, so a lot of shops are Perl-focused. If you use Puppet, knowing Ruby will help you extend it. I work at Google which is big on Python so I learned after coming here; it was a shock to the system after being a Perl person since 1991 (someone recently told me Perl didn't exist in 1991... I introduced him to a little something called Wikipedia).
From a career-management point of view, I think it is important to be really really really good at one of them and know a little of the others; even if that means just reading the first few chapters of a book on the topic. Being really really really good at one of them means that you have a deep understanding of how to use it and how it works "under the hood" so you can make better decisions when you design larger programs. The reason I suggest this as a career-management issue is that if you want to be hired by a shop that uses a different language, being "the expert that is willing to learn something else" is much more important than being the person that "doesn't know anything but has great potential" or "knows a little of this and that but never had the patience to learn one thing well".
P.S. Other thoughts on this topic: Joseph Kern has advice about the three languages every sysadmin should know and Phil Pennock has great advice and an interesting summary of the major scripting languages.