If you've ever been to an out of town business conference, you probably were greeted by someone when you arrived at the hotel conference room, or, if you are important enough, they may have met you at the airport. In many cases these are contract workers who work as corporate greeters. They are meant to make clients and potential customers feel welcomed, and they can make good money.
Greeters for major corporations can make $3,000 to $6,000 per contract, usually only working for a few hours total. If you want to be a greeter you have to be physically attractive, have a good voice, and be friendly. These jobs are sporadic, so you probably won't be able to make a living off them, but you can certainly supplement your income this way.
Other greeters work at grocery stores and movie theaters. These people make considerably less money, but they don't have to be good looking, friendly, or anything. In fact, I think those traits are frowned upon.