A good starting point for learning about certification is About.com's entry on quality assurance and software testing certification, which lists and provides links to several major organizations.
One thing to keep in mind: Any testing certification must present you with the basics of testing, but no single certification prep course can prepare you for every project you'll work on. The best thing you can do is take advantage of every training opportunity you can, read everything you can, and learn as much as possible on the job. If you're interested in knowing more, I have a series of articles on how to become a better tester on my personal blog.
This was first published in March 2009