Depends where your job focus and interests are. If you're working on webby systems, then figuring out python or ruby might be useful for your next job - a lot of the background knowledge is the same, but you need the language knowledge to be able to apply it. If you're really all about the systems, the a good knowledge of C would help when you need to change the bigger tools you're using for your own needs. Or for large system automation/integration stuff, look at make, no doubt you use it regularly, but I suspect you have little idea of what it does, how it does it, what it can be used for aside from making compiling C programs easier. Or there are plenty of other aspects that come from sysadminning, whether it's db development, db admin, networking, all can grow quite easily out of the same role.