How can an upstart watering hole lure ever-fickle customers? By having celebrity DJs? Exotic cocktails? Audacious art? David Sliskin, manager of the Alligator Lounge, pulled an even hotter idea out of the oven: From 6pm to 2am, free pizza is served with every drink. Word spread quickly, and now penny-pinchers from all over the borough make the pilgrimage for $10 pitchers of Rheingold and fine thin-crust pies, baked over a wood-stoked fire. At a time when little seems to rouse tousle-haired Brooklynites from their ennui, enthusiastic patrons fill the bar every night. And the DJs? Free pizza, Sliskin says, is the best DJ in town.