This could be the most dangerous thing in your house.
OK, it probably isn't, especially if you keep matches or firearms in the home. But hey, it's a good headline.
That doesn't mean you should close your eyes to the problems created by the ground lifter (seen above) also called a NEMA cheater. This little device, usually sold for under a dollar, lets you put a three-prong plug into a two-prong outlet. Most places have three pronged outlets but some extension cords don't. So you use this little doo-hickey, and go on about your way.
Except for one thing: that three-prong plug is there for a reason. At some point, some engineer determined that the thing you're using needed to be grounded. Not "should be grounded," he or she said "needed to be grounded." Adding a grounding pin (the correct term for that third prong) may only cost a penny to do but companies aren't going to do it unless they need to. So a three-prong plug is there for a reason. If you don't properly ground the thing, it could stop functioning right, even to the point of catching fire (although that's pretty rare, it does happen.)
The proper way to use a ground lifter is to run ground wire from that metal hoop to the nearest ground source. This could be the junction box in the wall or a cold water pipe. Or it could be the actual ground, you know, the dirt. If you're not doing that, you're doing it wrong.
For a quick fix, a ground lifter is probably not going to kill you, but if you plan on using one for a long time, do yourself a favor: use it right.