Bell Ringer: One who rings a bell.
In history, a bell ringer was the town crier, mayor, or (really) anyone else who had a bell and wanted to ring it. But we mostly think of the town crier and such.
Mostly we see bell ringers around Christmas (Hanukkah/Kwanzaa/Yuletide/etc.), ringing away outside department stores. Their main purpose is to collect money for a cause - for families who need Christmas money, for childrens' hospitals, and for anyone else who might need a spot of cash to get them through the winter.
The Salvation Army, for example, is often spotted ringing the bells in December. Their red color scheme (as well as the tintinnabulation of the bells) makes them easy to spot.
Some find it annoying (Edgar Allen Poe even wrote a poem about bells driving one mad), and others find it a good way to prove that they're generous. Then, of course, there are the people who really are generous, and those people make up the backbone of such donations.