The first resource that I found particularly useful was the Chromosome browser. This tool allows you to compare your DNA test to others who have been identified as DNA matches to you.
The goal of mapping is to use known relationships and their DNA to identify unknown relationships. So, I might map two cousins whose relationship to my family has been identified. Then I can add other DNA relationships to see if they triangulate, meaning we all share common DNA. This will then let me confirm from which branch of my family tree that individual is descended. The next step would be to compare family trees and see if I can figure out the shared ancestor. Several of the genealogy companies offer this service but I found this one particularly easy to use.
The second tool is called AutoClusters. This is an automatic tool that bunches your DNA matches into “clusters” of individuals who are likely to have descended from a common ancestor. In essence this tool does a lot of work for you in making matches. You have to request that this be done and then the results are emailed to you. I just did this and now I’m waiting so we’ll see how it goes. I’ll report back if I discover anything fascinating.