Prometric is a company which administers exams, notably the GRE and the Microsoft certifications (such as the MCSA/MCSE and MCP programs). Their main competitor is Pearson Vue.
Microsoft exams used to be administered by both companies, but as of December 31st, 2007 Microsoft exams were only to be offered by Prometric.
The problem with this is that Pearson Vue has a much higher standard for reliability at their testing centers, whereas Prometric will let a testing center with horrendously old and faulty equipment continue to provide testing, even when certain exam candidates (such as myself) have several exams crash during testing, resulting in lost time on the candidates' part and a rescheduling of the examination.
My personal beef with Prometric is that the closest testing center, and the only one easily reached by public transit, has failed to provide a proper examination all four times I have been there, twice requiring me to take a second day off of work in order to retake an examination.
This has led me to be more interested in taking the Cisco examinations, which are only offered by Pearson Vue, and I am finding them to be much more useful and realistic than the Microsoft examinations.