Nickel Metal Hydrid (nimh)
Warning! Charging batteries without due care can cause the battery to swell and even explode. The information presented here is basic and should only be used as a rough guide in combination with other sources. When in doubt, set your charge rates low and increase only through successful experience.
The most robust in my opinion of all the battery chemistries. Can handle heavy loads, many discharge cycles, and can be drained to nothing with little to no damage. High energy density second only to lithium batteries as far as mainstream batteries go. You'll most often see these batteries in AA and AAA consumer format but you can get industrial ones as well. Historically these have suffered from undesirable levels of self-discharge but newer models are greatly improved. Still, don't expect a full charge after a year in the closet.
One of the more difficult batteries to charge, nimh batteries are actually fully charged when the voltage begins to drop slightly. Unfortunately this drop is on the order of millivolts and can be hard to detect. The battery also gets warmer the more charged it is, so complex nimh chargers will use both temperature and voltage to detect full charge.
If you want to use the battery but don't want to build or buy a fancy charger, you can cheat a little bit by not charging the battery all the way. Find out approximately what the full charge voltage of your cell is (1.3 - 1.4 volts), then knock 10% off of that and you're good to go.
Another simple method of charging is called the trickle charge. This method involves very low amounts of current over long periods of time (days). Nimh batteries can handle a small amount of over charging with minimal ill effect. Prolonged over charging can result in a loss of battery capacity and an increase in self-discharge, so be careful not to leave the battery on the trickle charger all the time.