Some of the internal circuitry of a faulty Nikon SB-600 flashgun. Bought two of them for about £20 each, with the first being repaired in about four minutes - the time it took to clean up the mess left on the battery contacts by some leaky cells. The going rate for a decent one of these that works is about £60, so that gained about £10 per minute!

The above one needed a big strip-down to clean some unusual residue that was throughout the battery compartment. Like the first, it's now sorted, though that only gained about £10 per hour spent on it.

A general bit of advice based on experience with the above, and other equipment - don't leave any kind of non-rechargeable batteries in electronic equipment that's not going to be used for any length of time.