Whisk together the juice, zest, sugar, and eggs in a 2-quart heavy saucepan. Stir in the butter and cook over moderately-low heat, whisking frequently, until the curd is thick enough to coat a spoon, about 6 minutes. Strain the curd through a mesh strainer or sieve to remove any cooked egg bits. Transfer the lemon curd to a bowl, cover its surface with plastic wrap to prevent a skin from forming and chill at least 1 hour, until cold. Serve the lemon curd in tart shells, over pound cake, ice cream, or as a spread. Serving suggestions: Serve the lemon curd in tart shells topped with fresh berries, over toast or scones, as a filling for lemon meringue pie, or as a filling for cookies, cakes, or pastries.