You can substitute the Jell-O for sugar free, and the Cool Whip for Lite or Free. You can also use whatever Jell-O flavor (exclude the lime juice though.)
1 x 3 oz. box of lime Jell-O
16 oz. Cool Whip
1 x 10.5" graham cracker crust
2/3 c. boiling water
2 TB. lime juice
Pour 2/3 c boiling water over Jell-O in a bowl, and whisk for two minutes. Add ice to water in a measuring cup to equal 1 cup. Pour ice/water into bowl, and mix until cubes are dissolved. Add in lime juice and Cool Whip, and stir until smooth. Put mixture into the refrigerator for twenty minutes or until the mixture is REALLY thick and can be mounded into the pie crust.
After putting the mixture into the pie crust, refrigerate 4+ hours. We put ours in the freezer to make a frozen pie :)