Editor's Review of Affair Look
Affair Look is a discreet place where married people can find sex outside of their marriage. There is no fee to register and you can browse profiles for free.
Affair Look is from the same people as Adult Friend Finder, so the site is easy to navigate and offers plenty of search and communication tools.
I don't know how many of the women on the site were actually married, but my local search returned a large selection of attractive women looking for sex.