ScubaChuck has it right. In my experience most of the time you will normally see these tiny bets into large pots generally early in the game. As ScubaChuck noted most of the time the bettor has a weak hand and it is nothing more than a weak bet. Early in the game you still have the new riff-raff who really don't know any better and put out these mini-bets. If they are still hanging around in the later stages I would imagine they are still betting the absolute minimum then also.

What is very nice about this unless these players are putting on a show is that you can normally move them off of most pots with a modest reraise. Most often their power hands will be way overbet and they will be extremely eager to get all-in with monsters and not value or mini-bet made big hands.