Melt butter with lime juice in medium saucepan over medium-low heat. Let simmer 4 minutes. Remove from heat and add tequila. Return pan to medium-low heat and whisk until butter is beginning to simmer; keep warm.
Bring 3 quarts water and Old Bay seasoning to boil in heavy large roasting pan set over 2 burners. Add crab legs and cover tightly with foil; cook just until heated through, turning crab once, about 4 minutes total.
Divide crab legs among 6 plates. Divide warm tequila-citrus butter among 6 ramekins. Serve crab with tequila-citrus butter and garnish with lime wedges.