NiMH (nickel-metal hydride cell) batteries have been available on the consumer market since 1989 and are now one of the most common types of batteries available, with many variants including in the Duracell uplive top up batteries range.
NiMH batteries come in most common sizes such as AA, however C and D size batteries are harder to find, but there are adapters available so that you can use multiple AA batteries instead of a C or D size. Watch out for C and D size NiMH batteries for sale which are simply several AA batteries inside of a C or D size casing. Of course there are NiMH battery chargers readily available for these batteries too.
Pricing of NiMH batteries is similar to that of the alkaline alternative, with NiMH batteries rated at 1.2v and the alkalines at 1.5v. Having said this, many NiMH batteries, including the Duracell rechargeable batteries, will come relatively close to the 1.5v when new, or freshly charged. The big difference is that an NiMH battery in general will offer this output for a much longer time, whereas an alkaline, while it may appear to outperform the NiMH at first, will drop off in power at a faster rate.
Because of the more even power output over the life of the battery, NiMH has proven to be very popular in high drain devices which may requirement frequent battery replacement. These applications include use in digital cameras, other photography equipment and toys.
Lithium ion batteries are another option when you are looking for a rechargeable battery, as they are known for being able to often provide a higher energy capacity, however the life of the battery is significantly less than an NiMH alternative and are usually also at a higher price point. Apart from the Duracell rechargeable batteries NIMH range, I also recommend checking out the Sanyo Eneloop rechargeable batteries. These are what I am currently choosing to run in my photography equipment, mainly due to the very long lifespan of these batteries.