Swinging or partner swapping is a non-monogamous behavior, in which partners in a committed relationship agree, as a couple, for both partners to engage in sexual activities with other people. As a subculture, the swinging couple regard their sexual activities with other people as a social activity in which they engage as a couple. The phenomenon of swinging may be seen as part of the sexual revolution of the 1960s, which occurred after the upsurge in sexual activity made possible by the invention of the contraceptive pill and the prevalence of safer sex practices during the same period.
There are many swingers personals sites on the internet where swingers can meet one another. Some are even specialized, for instance http://www.couplesdeli.com is for swinger couples only.
The term wife swapping is now criticized as being androcentric and not accurately describing the full range of sexual activities in which couples may take part, but the term continues in use, and reflects the origins of swinging of actually swapping wives.
Wikipedia, Swinging, http://en.wikipedia.org/wiki/Swinging (as of Jun 14, 2010)