CeCe Moore is a professional genetic genealogist. Her blog breaks down the different tests and explains how to incorporate them into your research. She also lists great basic resources and recommendations for testing.
Roberta Estes, a scientist and genealogist, explains in-depth the various DNA testing services available and helps interpret results.
The Legal Genealogist
While Judy G. Russell mostly writes about the law and genealogy, she also does a series on DNA.
Other worthwhile blogs:
The Genetic Genealogist
Through the Trees
Genealem's Genetic Genealogy
And for even more information, check out the new book The Family Tree Guide to DNA Testing and Genetic Genealogy.