I will not be going into much detail regarding ESR or Equivalent Series Resistance Meters as, there is already plenty of other sources of information on the subject. Yet, every tinkering knows capacitors are guilty of a lot of sins in electronics. Capacitors love to throw red herrings! They can appear physically fine (no bulge), show good capacitance and hide in circuit, standing to attention like the Queens Guards hiding shorts and high resistance under their big hats. This is where the ESR tester can be a saviour, with the ability to test for "out of specification" high resistance, within the capacitor. They can also be used to test "in circuit", without the need to remove every capacitor in the circuit.
I included a few Easter Eggs in the Meter Scale!
Version 1.1 PCB
w2aew's overview of the circuit
Version 1.1 Schematic
Typical ESR Readings for low cost general purpose capacitors
Here is a tip, regarding using an ESR meter to check for faulty battery cells.
More information can be found on the "5 Transistor ESR Meter Design" forum thread on EEVBlog.My design files can be found on the Project Design Page here.
Just make sure you get a meter with the correct sensitivity in 50uA to 100uA range. You may get away with a different scale after removing any current shunts or series resisters from with in the actual meter.