18650 LiIon batteries are used to power flashlights and the like. I use them for inexpensive bike lights. There is a baffling array of 18650 cells for sale at a truckload of places. The difficulty is knowing which are good and which are not.
HKJ at Candlepower forums has done some excellent empirical testing - see the LiIon 18650 battery comparison and the 18650 battery test 2011 .
I have used the Trustfire 2500mAh blue label batteries for several years. The capacity is more like 2000mAh (instead of the claimed 2500mAh capacity), but they have proved to be reliable.
The best value for money / capacity readily available seem to be the Trustfire protected "true" 2400mAh batteries with the black and red labels .
Currently, the highest genuine capacity 18650 cells come from Panasonic with their NCR18650B 3400mAh batteries
Panasonic also have a series of 18650 cells with 3100mAh capacity