There are numerous certifications around. Almost all software giants who have well entrenched software products have rolled out a certification exam of their product - be it Microsoft, Cisco, Oracle, Sun Microsystems and others.
At one time, certifications were touted (at least in India) as a sure fire way to land a job in the IT sector. Many certifications such as those of Cisco are still valued and a CCNA, CCNP or CCIE has great value. Of course you might find a number of paper CCNA's around - ie those who have passed the exam by merely mugging up the question dumps provided by the training institute and no hands on experience on a router to show for it but I have heard that you cannot easily pass the CCNP and higher certifications of Cisco without solid knowledge about the networking concepts and hands on experience on Cisco hardware. On the other hand, Microsoft certifications does not seem to have retained this same level of credibility.
On the Linux front, there are a number of entities who have rolled out certifications. The most popular and well known among them being Red Hat which provides its RHCT, RHCE, RHCA certifications. What makes Red Hat's certification unique is that it is a completely hands on examination monitored by a Red Hat employee. And if you are a Linux enthusiast and aspiring to make it into the Linux world then an RHCE would definitely give you a great push as many corporate houses who make use of Linux invariably use Red Hat enterprise Linux.
Scott M has compiled an exhaustive list of questions and their solutions which he has encountered while taking his RHCE examination. I find these questions to provide great insight into what to expect in a RHCE examination.