National Stepfamily Resource Center
A division of Auburn University’s Center for Children, Youth, and Families, the NSRC's "primary objective is serving as a clearinghouse of information, linking family science research on stepfamilies and best practices in work with couples and children in stepfamilies."
Stepfamily Foundation, Inc.
333 West End Avenue, New York, NY 10023
"Our mission is to assist you to make the family-as-is-now work. We are the only organization to devote ourselves since 1976 solely to these issues." The blog is pretty good, and the books listed under Resources look good, too (all written by foundation staff).