Cedega/WineX (based off Wine) is more or less what you just described. It's not exactly free ($5 a month) but it plays most high-profile games with decent sucess. There's also a free CVS version, but from what I've heard it's not as up to date or complete. However, if you're looking to play a game that has never been that popular, good luck. It may work, but chances are some API feature isn't implemented yet.