Cigarette butts seem the most popular - though to be fair, I don't think they're aimed, it's just the usual casual, thoughtless disposal via window. Bits of food and wrappers also seem to take the same route. Evidently it's common to confuse the open side window with a bin. Ah, if I had a grenade, I'd be more than happy to reverse the process.
I suspect I've been lucky, the only thing purposely thrown is abuse. It's open window season. And not just cyclist-specific, shouting abuse at random pedestrians seems popular. I got called 'speccy' the other week, for which I suspect there is no comeback, as I was in fact wearing glasses. I say it loud and I say it proud, but I do suffer from myopia and I don't care who knows. A very astute observation from a Saxo driver. Someone also complemented my wife's breasts as we walked home the other evening. Or it may have been expressing the fact that he was "driving like a tit". I'm not sure of the attraction of shouting random abuse out of a car window, but then I have a mental age in double digits.
Schoolkids stepping in front of me is ever popular too.
Probably the most stupid incident I saw was someone throwing a full coke can out of a car window. It went straight into the windscreen of the car behind, cracking it severely and nearly causing them to lose control. The can also exploded fairly spectacularly. Some people are so stupid as to defy rational belief.