This takes a little forethought (a run to the hardware store before your trip) and the ability to find and navigate a hardware store in your destination country. But if you can manage those things, you can make yourself a very handy travel adapter for around $4 and it only takes a few minutes to put together. As an added bonus, you have one adapter with 3 outlets, so you can plug in your phone, the significant other's phone, and the kids' iPad all at once.
You can also use a similar approach for a 3-prong grounded adapter, if need be.
!!!!!NOTE: This is a plug adapter ONLY, it is NOT a voltage converter. So if you have 110V devices that cannot run at 220V (or vice versa), do NOT connect them to this adapter!!!!!
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Step 1: Before Your Trip: a Trip to the Hardware Store
Before your trip, stop by your local hardware store and pick up a cheap 6ft 2-prong extension cord. Home Depot has one for $1.57 (http://www.homedepot.com/p/HDX-6-ft-16-2-Cube-Tap-Extension-Cord-HD-145-017/100672781)
I wouldn't recommend going through airport security with wire strippers, so before you leave home, cut the plug end off the extension cord. I left the full length of the extension cord, because it doesn't take up much extra room, and the extra length comes in handy when the plugs aren't in the most convenient spot. But you can cut it to whatever length works for you. Then separate the two conductors so you have about 3/4" of split wire. Strip about 1/4" off the ends of both wires. Stick the modified extension cord in your suitcase, and Step 1 is complete.
Step 2: Find a Hardware Store at Your Destination
What you are looking for is a replacement plug end (see photos for the example from France) that will go into the outlets in that country. The example one cost 2 Euro. A 3-prong grounded plug end was 3.50 Euro.
When I visited France, I had success at a hardware store called Bricolage. You could also try Carrefour. If you find the right part at other stores, please feel free to leave comments as to where others might look. If you're visiting the US, you can go to Home Depot, Lowe's, or Ace Hardware.
Step 3 will require a screwdriver. I pack a pen knife with me, but if you don't have that, and you can't borrow one from your hotel's front desk, you should go ahead and spend a couple extra bucks to pick up a screwdriver while you're at the hardware store.
Step 3: Attach the Plug to Your Extension Cord
All that's left to do is attach the plug to your extension cord.
For the one that I am showing in the example, you slide the cover onto the wire. You then put the stripped wire ends into the holes atop each plug prong and tighten the screw.
Slide the main part of the wire into its track, and tighten down on the strain-relief screw.
Slide the cover into place (I did mention that it had to go on first), and you're ready to go.
!!!!!NOTE: I mentioned this at the opening, but it bears repeating. This is a plug adapter ONLY, it is NOT a voltage converter. So if you have 110V devices that cannot run at 220V (or vice versa), do NOT connect them to this adapter!!!!!