ZTS Inc. MBT-1 Multi-Battery Tester
This battery tester will test almost any battery that you come across. Some batteries will test good on a volt meter, but when you actually use them are dead. This meter puts the battery under load, getting a much more accurate reading of remaining capacity.
ZTS also makes a Mini version of this tester that tests the five most common styles of batteries, I chose to go with the larger version that test thirteen different styles of batteries.