I've seen "ugly tourists" of several nationalities during my travels, so it would be unfair to single out only Americans for this dubious honour. I try to remind myself not to stereotype people from a particular country, but rather to condemn bad behaviour (regardless of nationality).
Unfortunately, I'm ashamed to admit that a few of them have been my fellow Canadians. One that stands out is a young lady I encountered in the U.K. who spent considerable time "bitching" about just about everything - "why don't they drive on the proper side of the road", "why can't they do _____ like we do back home", etc. etc. It was an effort for me to keep quiet, as I was tempted to suggest that perhaps she'd be happier "back home" and don't bother travelling in future.
Of course, I've also seen several "ugly Americans" as well, similar to those that have been described in some of the replies here. Perhaps they're more noticeable due to the stereotype they have?
I've also encountered "pushy" people from Asia, inconsiderate boobs that don't respect the "No Photography" rules and arrogant Brits (I sometimes chuckle a bit with the Brits, as they make me think of the Vinnie Jones soccer hooligan in the movie EuroTrip. His tirade of profanity against the French when arriving in Paris on the double-decker Bus is always entertaining (esp. the outtakes, when he announces at the end, "I'm knackered" - it's on YouTube).
A somewhat "new" group in the "ugly club" appears to be wealthy tourists from Russia. I've spoken with shopkeepers in several parts of Europe over the last few years, and they've singled out that group as having a problem with being "rude, arrogant & pushy".
Too bad the "ugly" tourists aren't aware of Rick's philosophy to become a "cultural chameleon".
One parting thought - the majority of people that I've travelled with on RS tours (mostly American) have been wonderful tourists and very accepting of the local culture.
Cheers!