My previous 4.8 NiCd packs all had only a red wire and a black wire. My wife bought me a 1400Mah battery but it has a red, black, AND white wire. What can I use this battery for?

They may have just used 3-conductor wire like that on servos to assemble the battery instead of 2-conductor. As long as the polarity is right on the red/black wire, you can most likely ignore the white one.

i agree, i bet if you were to open up the pack and look at the wiring with a microscope you would see a bunch of bacteria sized lemmings jumping off of a white wire cliff. in otherwords i bet it goes nowhere, whatever company made it probabyl didnt want to bother with 2 different types of wires, one for batteries and one for signal dependent electronics, but instead just decided to use one wire for both, they probably save money that way.

Some systems (very rare now) required a center tap on the battery. If you have a voltmeter, check to see if there is any voltage between the white and black lead. If not is is just as stated above, the manufacturer using the same wire spool for batteries leads as for servos. In either case, just ignore it and make no connections to the white wire.