Ok, I'll share with you most of prejudices coming to my mind right now.
We italians have quite a culture in prejudice, there's even a political party utterly based on them (and they take themselves seriously). You can pay a visit:
http://www.leganord.org/ , i find it kind of funny. The less funny thing is they actually get a lot of votes. But on we go:
Brits:Poor personal hygiene, they tend to clean their houses quite rarely, and there're reports of new bacterial forms discovered in english carpets every day. And they all drive a Mini Minor.
French:Everyone has a baguette, and keeps it firmly in his/her/its armpit, even when wearing a tanktop. They love their language and tend to create a French term for every English worldwide-spread word. The highways have more toll booths than actual road, and they all drive Renaults, Citroens and Peugeots.
Slovenian:They come to Italy with the sole purpose of stealing workplaces from italians. They are generally plotting about hurting you, even if they're a community of slovenian nuns helping blind people to cross the road.
Germans:Every German is a blond hulking brute. They all drive either a VW, a Audi, a Mercedes or a BMW. Women tend to like s&m. they apparently speak with one-liners, and have a worshipping for bratwurst and sauerkraut. They laugh loudly and without reason. Beer is more common than water.
Kind of tired right now, but I think I'll write down those about americans-spanish-icelandics-italians when I'm back home tomorrow.