Some cameras take AA or AAA rechargeable batteries and I personally prefer this because you’re not locked into buying a proprietary battery and in an emergency you can buy batteries just about anywhere. They are heavier and the flash recycle time is longer when compared to lithium batteries.
The best current technology for rechargeable AA and AAA batteries is nickel metal hydride (NiMH). The trouble with these is that they have a high self-discharge rate, i.e. they rapidly lose charge just sitting on your shelf or in your camera. Then you go to use your camera and swear when the batteries go flat after five minutes.
The best NiMH batteries are a type called Eneloop, or one of the equivalents available with different names. The distinguishing feature is the fact that they’re charged and ready to use when you buy them. These have a very low self-discharge rate and Eneloop claims to retain 90% of the charge after six months (the conventional type would be dead long before then). I don’t know how good the other brands are, but I’ve been using Eneloop for a few years and they’ve been superb: they also seem to last longer in the camera when compared to conventional NiMH batteries.
All rechargeable batteries will die after a certain number of recharges, and the recharger that you use makes a big difference here. The type of charger that just charges for a fixed time period will overcharge your batteries and shorten their life significantly. The best type measures the charge in the battery, temperature, and other factors to give your battery just what it needs and no more.
I’ve been using an Energizer CH1HR for about 2 years and had no trouble with it. It’s a one-hour charger and gives a trickle charge to maintain the battery after it’s charged. If you find the appropriate cord you can also use this charger in a car. Click here to view a data sheet (PDF 54 KB).
A final note: I keep my NiMH batteries in the fridge between uses. Eneloop’s information says that the batteries last longer when stored at low temperatures, but must be kept dry. Eneloop also says that the batteries can tolerate a storage temperature of -20degC and be used or charged at 0degC, so refrigeration is no problem.