Ingredients

1 box cake mix - any flavor
1 can diet soda - any flavor

Variations I've seen:
chocolate cake w/ diet coke
yellow cake w/ diet lemon-lime
use your imagination for the combinations!

A suggested frosting:
Cool Whip Free w/fruit
Cool Whip Free (8 oz) mixed with 1 pkg. dry sugar free cocoa mix powder
(be sure to calculate your units for frosting)

:

Mix the dry cake mix with 10-12 oz of diet soda. (Do not use the egg or oil listed on the box). Spray a 9 x 12 cake pan (or cupcake tin) with non-stick spray and bake according to package instructions.
Some have said an entire can of soda makes the cake TOO moist ... 10 oz. seems to be the least I've seen suggested. Experiment yourself to find the best amount for your taste.
Also ... check the fat content on your cake mixes ... some "reduced fat" have apparently had higher grams than regular ... store brands sometimes lower ... take the time to compare