Affairs Club is a site targeting married people looking for an affair. The difference is that the site actually was around from way before the niche got hot and popular.
As a pioneer in the "married dating" space you have to give them credit for taking all the heat when it was extremely taboo and not accepted. Now that its almost chic, Affairs club is worth checking out. It doesnt have the polished look as some of the newcomers but it has a database of members that gives it a lot of value.
The site is easy to navigate and uses the same privacy, discretionary features as to not reveal your identity if you wish to keep it private.