One player to a hand, a fundamental in poker, is more complicated online. Everyone agrees chatting during a hand, about the hand, is as big a no-no in a virtual game as it is live. Beyond that, the mystery and threat of collusion is persistent Players are in agreement that actively colluding at a table is simply cheating.
Sharing cards via phone or IM is another form of cheating. Coaching during play is a grey area. Deep in tourneys, a backer or coach may be observing play and providing input. There is nothing officially illegal in this activity; however, it goes against the spirit of play and my own barometer for what is proper. How would the best casinos and card rooms handle this? It is not allowed in live play, it should not be practiced online. I have observed a player in online ring games and then debriefed their session with them, capturing their questions during play as well as the cards they are playing. Having someone to discuss your play can aid in identifying leaks and strengthening your play.
Chip dumping in tournaments is a form of collusion and one needs to be aware of this possibility. This occurs live as well, but the sheer volume of online tourneys makes the play more common. The play: Player A raises, Player B with a large chip stack re-raises, everyone goes out, Player A moves all-in, and Player B mucks rather than calling the small raise. Both players now can move ahead with a decent stack, improving their chances of cashing and moving up. Chip dumping goes against the spirit of play and should not be condoned. If you suspect this is happening, you must report it to the online site for your own protection and the integrity of the game. Most sites will investigate this type of play extensively to determine if it has happened previously.
By far the most common etiquette abuse online is cursing and abusive language. The chat feature of an online table is a way to reach, literally, to the other side of the world in conversation with the other players at the table. Too often, players verbally attack other players with disparaging remarks. These comments fall into a few categories: "...you are a horrible donkey!" - "I can't believe you called with those cards, moron," and other various threats. For some players, these are emotional outbursts caused by losing a pot or series of pots. For other players, these abusive remarks are a calculated strategy to loosen up play at the table. It is often a very good strategy as most players assume that the abusive player is a reckless target to attack. Don't be sucked into these diatribes. You'll profit in the long run by keeping your emotions in check. And, once again, you can report them to the online site and/or turn off the chat from that particular player.
Additionally, if players are playing loose or making poor decisions, why would you want to correct their play? So that they can play better? It's painful in the short-term but profitable in the long run. If you feel rage and tilt building, it's best to either move tables or take a break. Get out of the house, wash your face, get a snack. You'll be in better shape to make the right decision rather than getting in a typing war with the villain.
Probably the worst breach of etiquette during play was the disconnect. If you haven't seen this play at some point, then you may be fairly new to online poker. It goes like this: there is a raise to you and you call with 7-7 and have another caller. The flop comes Ah-9d-7h with two hearts. The pot has $45 and you have $495 to the villain's $570. You bet the pot, and he bets $150. You then move all-in for another $345 into the $240 pot. The seconds go by, and then the 15 second clock starts ticking down. After hitting zero, you realize that he has timed out on you. Jd and 2d gives his Ad-Kd the pot, back-dooring a flush without his having to make the decision to call or muck his hand. This play is rare and unique to online poker; it is fundamentally shooting an angle. Repeated abuse of this play used to see a player lose their disconnect protection privileges. It is a no-class move and has no business at the virtual table. Very few online poker rooms allow disconnect protection now which prevents this from happening.
Whether you're playing micro-stakes or playing in the biggest online games available, you have a responsibility to abide by proper rules and etiquette. The game will be better for it, and you'll see +EV.