An article on the SBNation.com web site reports that the Cincinnati Reds lead the league in the number of self-serve beer taps they have in the ballpark.
The way this reportedly works is that you pre-pay for the beer and have your ID checked. You're given an RFID-based card. When you are ready for a beer, you go to one of the dispensers and tap your card on the spot indicated on the machine. Then you can dispense your beer from one of the machines' taps.
There will reportedly be 80 self-serve taps at the park this year.