This is from allrecipes.com. I added some berries to the top (strawberries once and blueberries once) and it turned out fantastically delicious.
4 cups chopped rhubarb
1 1/3 cups white sugar
6 tablespoons all purpose flour
1 tablespoon butter
1 9-inch double pie crust
Preheat oven to 450 F.
Combine sugar and flour. Sprinkle 1/4 of the mixture over bottom pie crust in pie plate. Heap rhubarb over this mixture (topped with added fruit, if applicable). Sprinkle the rest of the sugar and flour mixture over the rhubarb. Dot with small pieces of butter. Cover with top crust.
Place pie on lowest rack in oven. Bake for 15 minutes. Reduce oven temp to 350 F and continue baking for 40 to 45 minutes. Serve warm or cold.