My favorite is the Simprop on-board battery checker. It is the size of a postage stamp and is about 3mm thick. It has 5 bright LED's that graphically show the battery voltage. I have one on each helicopter. In the above photo the simprop circuit board is stuck on the gyro control box.
Several companies sell battery cyclers.
Some of the devices on the market now are very sophisticated computer controlled multi-use chargers. A battery cycler can be helpful to determine how well your battery is holding a charge. Each time you cycle your battery, keep a record of the discharge time. Cycling a battery is the only way to determine how much charge is in the pack. The meter shows that battery is ready to fly, in the green, but we both know that it wouldn't last long when you try to fly it.