These are more popular rechargables, they also come in 'standard' sizes. These are a good replacement for standard alkaline batteries in many cases. The battery cel voltage is 1.25V per cell, that's less than the 1.5V of alkalines but more than the 1.2V of NiCads. The most annoying thing about them is their high self-discharge although battery technology has improved and there are a few low-self-discharge batteries on the market. They like to be charged at about 0.1C but can be discharged at 0.2C
Pros: Good alternative to Alkalines in most situations, high power density, "standard" size, better capability than alkalines, pretty easy to recharge but not as rugged.
Cons: More expensive than Ni-Cads, service life isn't as long, self-discharges quickly.
Prices: AA size battery costs around $2 and has up to 2500mAh of charge capacity.
Power Density: 100 Wh/kg