If you find that you have no baking powder and plenty of baking soda, just add in about a tsp of vinegar with your tsp of baking soda. Baking powder is just a powder that combines a basic (soda) with an acid (cream of tartar) and a filler. Vinegar works quite well to activate the baking soda, I've found.
This post brought to you by my day in PJs and my compulsive need to bake at 11 pm. Sure, I could go to the grocery store, but isn't it more fun to play chemist?
side note: It's not quite as good as using baking powder, so I'd only use this tip in a pinch. Leaves a touch of an after taste, I think.