Okay, there are a lot of different chess etiquettes to follow, and it depends a lot on your milieu. If you're playing a friendly game of bar chess with a friend, or in a tournament with a professional, or even with a random person here on chess.com, you behave differently. The anonymity of online gaming tends to influence people's behavior quite a bit, often in a very negative way, so I'll address mostly online chess related things. Mind you, I am not a professional player, I don't always have the right answers, I only have my own opinions, and lucky you -- I am willing to share them! ;)
When to Quit?
I have read several discussions about when the right time to quit is. Looking at the top-ranked players, many of their games end with a resignation once they realize that they can't win. For amateurs like myself it doesn't always make sense, but then again I'm not able to look 15 moves into the future and analyze games in my mind like a serious chessaholic can. Professional players generally don't make mistakes that will tip the game back in your favor once you've lost your footing in a game, and this is understandable.
For the amateur player, however, you will often find that just because you're down 3 officers and 4 pawns to the other player, you can eke out a win from left field. While I have resigned from games where it is clear that I have no ability to win (i.e., I have my king left versus the opponent's king, queen and a bishop), even in those situations I have been known to try to trap the opponent into a draw instead of just tipping my hat and my king to the opponent's superior play.
When I play against anyone, pro or a pleb, I don't expect anyone to fold just because the game appears to be going in my favor, or even if it's pretty much a guaranteed win for me. If people want to drag it out until the very end in an attempt to get a draw out of it, I'm happy to oblige. While trapping the king in a corner with a knight and a king is tedious and time consuming, even this very end part of the game can give you some insight into the workings of the board and your pieces.
If you don't have anything nice to say, don't say anything at all.
We were all taught this as kids (or most of us, I hope), and it's as true online as it is in real life. Just because you won't suffer a punch to the nose when you get cheeky online doesn't mean you should act in a disgraceful manner. I have seen many an online discussion where people resort to petty name-calling, threats of violence, and worse. A lot of these people are "trolls," merely trying to incite anger or redundant discussion, and the quick and easy fix for this is: don't feed the troll. If someone says something obviously outrageous like "I won't play against someone if I know they're gay/black/hindu/whatever," you can be pretty much guaranteed that they're simply trying to egg you on into an off-topic discussion which is likely to turn into a flame war between people who never had any issues with each other. Leave these posts be, ignore them and move on.
As for actual game-chat, I've been known to throw some e-dirt at people I play with. However, these are people I know and see in real life, and people who know I'm joking and throw as much dirt right back. With strangers in random games, however, if I say something at all it is likely to be friendly (or at least civil). Many players don't like to chat at all, and will simply not respond to your chatter -- when this happens, take the hint and just play the game. There are plenty of sites dedicated to social networking, your chess game doesn't have to be an extension thereof.
People that don't speak my language.
There are players on here from all corners of the world. Many (or even most) of them speak English, some of them speak terrible English, and some none at all. Just because someone has poor grammar or spelling doesn't mean you have any right to make fun of them. Just like people with a foreign accent, not only do they speak a language you likely don't, they speak a second one (even if it's not as well as you do). That's the beauty of chess -- it transcends language and culture. Enjoy it.