I don't know about any free one's that are like totally recognized, but
www.brainbench.com has a Gerneral Linux test there for free, and I think they give you a certificate if you pass. They use to be totally free on all tests, last time I checked though they cost money, but I do know they still have the Linux Admin test for free. I think they make you pay for the certificate if you want it after passing though, or they make you pay shipping charges.
