I agree Commoner chewwing gum is disgusting, in our town they now have little shelf thingys for people to dispose of the used gum. It still looks horrible but at least the stuff doesn't get stuck to your shoes
coccinelle - I've noticed cans and bottles adorning trees as well. Bizarre, and still littering. Probably not as bad as the people who carefully put their dogs' poo into plastic bags and then chuck it into trees or hedges. Not much fun if you're out walking in a high wind.
It really annoys me when I see people dropping litter, if I think I won't get a smack in the mouth I'll day something to them.
Also people who chuck stuff out of the car window, couple of *** at work always do this and then come out with the old classic "I'm keeping road sweepers in a job"