I am 41, female, married with two kids. My husband and I started playing together a couple of years ago. We've had a steady girlfriend that we would have sex with regularly, and a few girls we've met with a couple of times for drinks out and then sex. I'm not a jealous person, but would not be happy if he ventured out on his own without me, but when we're having fun and playing together, I share him without any hangups. I'm interested in the possibility of taking this further and actively looking for a woman to share our marriage with long term. Do you think this is possible? and maintain a healthy marriage?

A woman will eventually turn into a WOMAN, no matter how open minded she seems at first. So if you let someone in to share and she seems open minded and cool, it won't last long. She will inevitably start to think, plot, and cause drama. You have to be very picky and really know who you are dealing with, background etc. Just as an idea, you might want to look at another culture. People from other cultures may be truly more accepting to this and more easily fit into the role.
Things you may not have considered: Are you only interested in polygyny (>1 wife) or would you like to try general polygamy or polyandry (>1 husband)? Once you have a third partner, do you intend to close your relationship boundaries (polyfidelity) or keep them open? What if your new partner has a different view?
I recommend getting a copy of "Opening Up" by Tristan Taormino. It's very helpful in finding out all the different choices that you will be faced with and may have never considered before. You might also enjoy "The Ethical Slut", but my wife found it anti-monogamy and a bit angry but that may be because she is definitely monogamous by nature.
ps. You are normal. You are not evil.
If this is the kind of activity you like to engage in, it would have been wise never to have gotten married, especially not have kids who are highly at risk of becoming hurt by your behavior, should it ever be revealed to them somehow. And if they already know about it, then you are facing a catastrophic train wreck down the road from here.
Remember that your children will one day turn out better or worse than you, all based upon the moral foundations you taught them by.
I suggest you seek help and fix this problem before it's too late. Before the consequences bare a price too steep to pay.
As the above poster stated, there's nothing inherently wrong with it, it's just considered culturally/morally wrong. Maybe you should take your question up with Dan Savage (writer of the Savage Love advice column). He's good with advice for things like this, and does research when need be. :)
Good luck to you!
goodluck