I'm so glad to see that Sanyo finally delivered what I really want, rechargable batteries that work when I want them to. Often I want them to work weeks or months after I last charged them, but most NiMH (Nickel Metal Hydride) rechargables self discharge pretty fast and there is no charge left when I need it. With the eneloops, you can charge them after you last used them and chances are there will still be charge left weeks or months later.
I've put them in flashlights and camera flashes, both items you don't use often, but you want them to work when you need them. And I've very happy with their performance and shelf life. They come pre-charged from the factory so you can even start using them right away. The self discharge rate is dependent on ambient temperature, so they will hold a charge longer when stored in cooler temperatures. I'm not sure they are what you want if you leave a flashlight in your car trunk all summer, but they are perfect for things like flashlights, remote controls, cameras, clocks and other things which require a low self-discharge rate.
With the price of non-rechargable batteries and the toxic materials they contain, it only makes sense to use a battery which is capable of being re-used a number of time. I highly recommend the eneloops.
I've seen them available both with and without a charger. You can use any 1-5 hour battery charger to charge them so if you already have a charger, you can probably use it. The eneloop charger is a slick white thing which matches the battery design and it is not that expensive. High speed chargers (1/2 hour) are likely to create a lot of heat in the cells - that is not usually a good thing.
They are available in AA and AAA sizes.