eBlaster, from what I understand, is virtually undetectable, and he probably won't even try to do any deep registry checking if he thinks YOU are not tech-savvy. It's less than $100, and works quite well.
There are also lower-priced, and even free ones out there, but I'm not sure how hidden they are.
I do tend to agree with you, however, based on what you've told us about your husband -- he'll probably just create another account. But even so, I DO think you need to ask for (demand?) full transparency as part of your family therapy, if nothing else than for a strong moral stand for your marriage. If he wants to break that, by secretly creating other accounts, that's his choice.
With a history of past infidelity, I always suggest having at least ONE piece of intel in place that is unknown to the formerly-cheating partner: either a keylogger, or a voice-activated recorder, or a GPS.
