(or diet "soda" depending on where you're from! LOL)
I think someone had told me you could do this a long time ago, but I forgot about it until someone just mentioned to me again the other day.
You can use a dark diet soda in chocolate cake
lemon-lime diet soda in a vanilla cake
ginger ale or root beer in a spice cake
etc, etc.
I just dumped my cake mix in a bowl, then added 1 can of soda. Mixed it up and popped it in the oven!