This one is about as easy as it gets, and the result could not be better.
Having moved into a home with no doorbell, we wanted to ensure we could hear the ring from the basement and back yard. I did some research and found plenty of multi-chime systems, from cheap to expensive, but I knew this had DIY written all over it.
Unrelated, we bought a wireless phone set which can connect to bluetooth, so my wife and I can receive calls to our mobiles and pick up anywhere in the house. This particular unit (VTech) also has a "push to talk" to other handsets, and a full Intercom mode which is more like a normal phone call. Unfortunately, both involved (a) taking the handset off the charger and (b) either holding the push-to-talk button or going into a submenu for Intercom.
I was going to try the push-to-talk feature, when I discovered the "Find Handset" button on the main base unit. It made a very nice chirp in all three of the handsets, which lasts a minute, then automatically stops. Perfect!
After some not-so-minor wrangling to get the case opened (there were some snap-overs which did not want to budge), I was able to solder some small-gauge speaker wire across the button's contacts, and thread it out to a nice handsome $5 button outside. We have a through-wall mail slot, which made it easy to get the wire out.
I imagine this could be done with a wireless doorbell setup as well; just a matter of getting to the receiver to make a similar jump.
Test video here:
Installed video here: