While I don't like the shift away from free parking for all at MGM properties, they are now offering a MasterCard which includes free parking as one of the benefits.
It's a no-annual-fee card and there are some other perks and points system.

Here's LVRJ article about it --
http://www.reviewjournal.com/business/casinos-gaming/members-qualify-free-parking-mgm-s-new-m-life-rewards-mastercard