Rayovac AA NIMH battery charger.
2-battery or 4-battery charge mode.
I don't know how I'd even begin to total up the savings this battery charger has brought me. It's paid for itself many times over. I take a lot of pictures with my digital camera (not very good ones - I admit) and, as anyone with a digital camera knows, they drain batteries pretty fast. This particular AA battery charger cost twenty bucks at Walmart and came with four rechargeable batteries. I believe there's also a 12-volt car lighter adapter for using the charger in a vehicle, but I don't have it.
The 2-battery or 4-battery charge mode obviously means you can charge either two or four batteries at a time. I'm sure this is a standard feature on any battery charger of this type, but this is the only one I've ever owned so I'm sort of impressed by it. While I'm using two batteries in the camera, two are on the charger, and vice versa. With even a minimal amount of planning I can always have two fresh batteries.