Light strings work on a circut, which means that power has to go through each bulb to light the next one. Each bulb has a shunt, if the filament in the bulb burns out, power is supposed to bypass the blown out bulb and go to the next one, but these shunts don't always work, and no one wants to sit with a 50-string of lights and replace each bulb until it lights up again.
Saw this at my fave garden center, Light Keeper Pro is a tool that allows you to fix a blown out light string by simply taking out any bulb, placing the empty socket into the tool (which looks like a gun) and clicking a few times, (this makes the shunts activate) the lights come back on, allowing you to see where the blown out bulb is located, so you can replace it. You don't even have to take the lights off the tree. (you can also test fuses and larger sized bulbs with it)
Since I end up throwing/recyling light strings that probably only need a new bulb, I'll probably pay for this tool in one year. This goes to a site, but just google the name and you might find it cheaper.
http://christmaslightstore.com/Light-Keeper-Pro-Repair-Tool-505040.htm