This month's "Why didn't I think of that" award goes to Panasonic's nickel-metal-hydride (NiMH) battery set, which converts AAs to C or D batteries. It comes with rechargeables, but alkalines work as well. Just pop on the appropriate jacket and you're ready. Of course, you get the battery life of a AA, not that of longer-lasting Cs and Ds.
Price: $30; includes six AAs, two AAAs, four adapters, and a charger.