this recipe is not mine (shock!), but a friend's -- who gave me permission to share -- and it makes a fabulous pie! it's a wee bit different from rtebalt's so I thought I'd pass it along...
Key Lime Pie II
4 teaspoons grated zest plus 1/2 cup strained juice from 3 to 4 limes
4 large egg yolks
14-oz can sweetened condensed milk
11 full graham crackers, processed to fine crumbs (1 1/4 cups)
3 tbs sugar
5 tbs butter, melted
3/4 cup heavy cream
1/4 cup confectioners sugar
1/2 lime, sliced paper thin and dipped in sugar (optional)
For the filling: Whisk zest and yolks in medium bowl until tinted light green, about 2 minutes. Beat in milk, then juice. Set aside at room temperature
to thicken.

For the crust: Adjust oven rack to center position and heat oven to 325
degrees. Mix crumbs and sugar in medium bowl. Add butter. Stir with fork until well blended. Pour mixture into 9-inch pie pan. Press crumbs over bottom and up sides of pan to form even crust. Bake until lightly browned and fragrant, about 15 minutes. Transfer pan to wire rack. Cool to room temperature, about 20 minutes. Pour lime filling into crust. Bake until center is set, yet wiggly when jiggled, 15 to 17 minutes. Return pie to wire rack. Cool to room temperature. Refrigerate until well chilled, at least 3 hours. (Can be covered with lightly oiled or oil-prayed plastic wrap laid directly on filling and refrigerated for up to 1 day.)
For the whipped cream: Up to 2 hours before serving, whip cream in medium
bowl to very soft peaks. Adding confectioners sugar 1 tbs at a time,
continue whipping to just-stiff peaks. Decoratively pipe whipped cream over
filling or spread evenly with rubber spatula. Garnish with optional sugared lime
slices and serve.