One of the more "reputable," and possibly the one intended, is at National Geographic Genographic Project.

This study makes no claim of being able to trace individual ancestors, but only intends to relate your DNA to probable migration paths within the population. It may tell you where you come from but makes no claims about being able to identify who you came from.

There are a number of "commercial" sources for DNA analyses that make varying claims about geneological information they can find, but this field appears also to include at least a few "less credible" operators who just want your money. It has to be remembered that they can't tell you anything about specific ancestors unless they also have the DNA from the specific ancestor or from close descendants of him/her.

The Nat Geo survey looks good enough that I'd be tempted to participate "for the good of the science" but as it traces the maternal line, costs money¹, and my sister (who almost certainly has the same mother I did - we're not as sure about dad, maybe) has sent in her sample, so I've decided to defer my participation.

¹ The Participation Kit costs U.S. $99.95 (plus shipping and handling and tax if applicable).

John