If you have a device which uses AA or even AAA cells, you can purchase Nickel Metal Hydride (NiMh) rechargable batteries. They were one of the first kinds of rechargable cell to be hailed as having "no memory effect" (although in reality, this means "less vulnerable to memory effect"). While not as spiffy as Lithium Ion cells, they do hold considerably more charge than Nickel Cadmium (NiCad) rechargables. They can be charged in most existing NiCad chargers, only the charge times will differ (longer).

PS. Follow the NiMh link for information on where you can buy them.