Like it or not, companies like certifications. Cert's help HR department figure out baseline skills. They make for an easy box to check on an hiring request. It is far more difficult to interview and discover abilities than it is to look for some acronym of ability.
Need another reason?? Learning is fun or at least should be. Especially if your a full time geek. I am amazed on how many new things I learn just looking at some of the prep information. Besides, some of our favourite commands evolve and morph all the time.
That being said, one very good organization is the Linux Professional Institute or just LPI. I'm not saying it's a Cisco cert or RHEL cert but it's a good all around way to get a little more on the old resume. Besides it's a bunch cheaper to get than a lot of the others.
What's even more interesting is that IBM has a very large amount of helpful training information geared directly toward passing LPI group of tests. And best of all, the price is right... free.
The main starting point for IBM is HERE
LPIC-1 certification prep.
LPIC-2 certification prep.
LPIC-3 certification prep.