Linux Professional Institute, they have a series of exams, non-distro specific (so basically based around LSB (Linux Standards Base).
The first exam is really easy, but quite vague, I have an ongoing discussion with the UK moderator regarding the wording of some of the questions.
From a commercial perspective, though the LPI exams are not readily recognised, however the RedHat ones are (RHCSA, RHCE, RHCA etc), but they are a) more expensive ($450 for the exam alone) and b) harder. However, as the majority of major commerce use RHEL they are of more use (in my experience).
As a side note, as a Linux contractor, I run a mile if the customer uses Ubuntu,Gentoo and even CENTOS as it tells me they are not serious about their Linux infrastructure.