White Rock Lake has an annual picnic/festival w/ an all you can stomach of crawfish. I went a couple of years ago and it was a real task to wait in line and eat them in less than perfect circumstances. (it was a very windy day). The two times I went back for more crawfish I ended getting the 'bottom of the barrel' mudbugs which looked as if they were still covered in mud and crud from where ever they came. (most likely a farm, but still disgusting to see crawfish w/ a brownish/black goop all over it)
Grapevine bar off of Maple (about two blocks north of Oak Lawn) has a party called Suckfest, where they grill burgers and hot dogs and offer all you can eat crawfish as well. And it was all free last year. The only problem was the place was packed and we ended up sitting on the concrete (in front of the basketball hoop) and scarfed them down. Long lines as well. But, hey, it was free and good.
Don't know the exact dates of the two events listed, but they are coming up soon.