Bottles, cans, an ice ball, spit, and other debris. Haven't been knocked off the bike yet, and never got revenge on any of these cowards.
A girl I know was hit in the chest with a grape Slurpee (slushy drink) and her riding companion caught up to the idiots and got them arrested.
My ex wife got slapped on the butt once and it almost made her fall. When she got home she showed me the red outline of a hand still visible on her skin.
If I had a rocket launcher.....