Cigarette butt was about the only thrown object I can think of, but lots of close drive-bys, especially from large pickups. But those people are hazards to everyone, not just bikes.

I've become very communicative while on the bike -- I try to signal drivers when I've got to move into the lane to avoid an obstacle, and I wave thanks if they have to slow down to wait for me. I try always to signal my turns and I try to wave to the intermittant traffic out on country roads away from town.

As they say, courtesy doesn't cost much. I figure the more good will I can spread, the better for all.