Ooohhh, ooh, ooh! This is my area of interest, and my professional specialty.
Some good ones have been brought up already, but I have some to add.
In Chinese culture, gift giving is a very intricate, very important, and very common practice. Often, though, when it comes to cross-cultural interactions and gift giving, the presentation of a gift (for example, to a boss) could be interpreted as a bribe. I work in a school with many Chinese students, and sometimes, when parents bring gifts to parent-teacher conferences, teachers misinterpret this gesture as a bribe. This is a very common cultural misunderstanding.
In Japanese culture, slurping your noodles shows that you've enjoyed your meal. In American culture, slurping anything while eating is considered very, very rude.
In many cultures, when a child is reprimanded they may smile (vietnamese comes to mind, but with a little research you'll find this is applicable to many cultures), indicating that they are sorry. This is often misinterpreted, especially in today's schools. In the states, if a child smiles when they are being reprimanded, it means they are not taking the punishment seriously, and would invoke more anger in the adult.
Along the same lines is the difference in eye contact between children and people they respect, ie. teachers, elders, authority, etc. Again, the lack of eye contact is not received well in the states, especially in schools. A teacher will often say "Look at me when I am talking to you.", all the while not realizing that the child not making eye contact is them showing respect to you as they know it.
In many cultures it is rude to show the bottom of your feet (Turkey, Thailand [in regards to the Buddha]...).
In Japanese culture it is customary to take your shoes off at the door. Shoes are not worn inside.
In many cultures it is considered inappropriate, rude, or bad manners to hand things using the left hand. Things should be passed only with the right hand.
When I visited China this summer I was shocked at the lack of line formation. Often times people will "cut" in line, and its considered nothing there. I noticed that people would wait in front of a stall for the bathroom, rather than making a line at the door and waiting for the stall to become open. Another thing that surprised me was that in fast food restaurants (ie. McDonald's), or a place like Starbucks, you don't clean your table. There are people whose job it is to clean your table, and if you do it yourself, its considered rude, as you are taking their work away. With both of these examples, if this happened in the states it would be considered very rude, or a visitor in China may be confused by this, but its all just about understanding that what is considered normal behavior (a social norm) is just different in each culture.
Many different countries have different tipping practices. Tipping does not happen in many Asian countries. While staying in Japan and China I did not tip at all. Never. For anything. It wasn't customary, in fact, locals prefer that you don't tip, so that tipping is not introduced, and then expected, in their country. In Thailand, however, we tipped. Because of the presence of tourism in Thailand, tipping is now customary, in fact, tips are often included in the bill.
Last one, I promise! The attention paid to time is very different in many countries. The way that Americans value time, place importance on scheduling, and the phrase "Time is money", for example, is very different than, say, South American countries. Misunderstandings can often happen when it comes to being on time for a meeting, for example.
Its really important to note that not every cultural generalization or stereotype (not all stereotypes are negative) applies to each person of that culture, and some of these generalizations apply to many countries/cultures.
Sorry to go on and on! I hope this helps!!!