You don't always have to go out and buy a new one- particularly if you have arrived in the UK. In the UK you can use a pen or plastic pointy object to open the bottom middle opening (NOT one of the two prongs), but the third/bottom hole which is just an earth wire.
This opens the slots for the two top holes and any US plug will fit in and should work - and maybe even some Australian plugs. Always research and double check the voltages and amperages of the device you are about to use & NEVER stick metal or anything into the top two live holes!
Written by Peter Cain
Peter Cain is the Founder and CEO of dataGO.co, a company committed to making it easier and cheaper to stay connected whilst travelling. You can find him on Twitter and Google+. You can also find dataGO on Facebook, Twitter, LinkedIn, and Google+ for all the latest info on local and roaming SIMs and data hotspots.