Like this one:
This. I actually just purchased this same adapter and received it in the mail yesterday. It seems to work well most of the time, but sometimes I'll need to unplug and re-plug the thing back in for my MacBook Air to recognize my external mic. The audio in the right side of my headset also sometimes gets cut-off and I'll have to twist the 3-pin headphone end a little for that side to be audible again. But it's not bad for $11, and I do recommend it. Perhaps a similar adapter from a different brand will be more reliable though.