I'm almost ashamed to admit it now, but yes, I have on a number of occassions in the distant past. But I would never do it again.
I never once initiated the relationship, and always fell for the "my husband doesn't appreciate me", "we're separated/getting divorced", "he doesn't care" lines just to make me feel better about it. I have to admit that I did get a thrill from the forbidden fruit aspect of it.
But that was long before I got married, and being happily so now find it completely distasteful. During my single phase between marriages I was propositioned by two of my ex-wife's married friends, but fortunately had the presence of mind to say no, although the absence of nookie makes the mind strain to do the opposite.
Even now I struggle to try to send the message that I am happily married and uninterested in extracurricular affairs, pardon the pun. I even got propositioned by the husband of one of my wife's acquaintances at the college not long ago! He took the rejection like a gentleman, I must say.