Netiquette

Netiquette primarily involves any communication you may send out or receive from the net.

This short summary has been largely adapted from the Netiquette Guide-lines RFC:1855. These are recommendations, not rules:

If your Internet access is through a corporate account, check with your employer about their policy regarding private e-mail.
Don't assume any Internet communication is completely secure. "Never put in a mail message anything you would not put on a postcard". Likewise, independently verify any suspect mail, as addresses can be forged.
If you are forwarding or re-posting a message, don't change the original wording.
If you are replying to a message, quote only the relevant parts.
Never send chain letters, they are forbidden on the Internet. Notify you System Administrator if you receive one.
Do not send abusive or heated messages (flames). If you receive a flame, it is best to ignore it.
Take care with addressing mail.
Allow time for mail to be received, and replied to, keeping in mind time differences around the world and other people's busy schedules.
If you want your mail to be read, don't make it too long unless the receiver is expecting a verbose message. Over 100 lines is considered long.
Remember the Internet is a global community, and other peoples values and outlook on life may be different to your own. Be tolerant and careful with slang or phrases that may not be understood in another country.
Use mixed case, UPPER CASE LOOKS AS IF YOU'RE SHOUTING.
Mail should have a subject header that reflects the content of the message.
Unsolicited e-mail advertising is unwelcome (and forbidden in many countries).
When attaching files, don't send any larger than about 50k.

Unlike one-on-one e-mail, a large number of people read newsgroups and subscribe to mailing lists.

It is always a good idea to read what others in the list or group are saying for a while, to get a feel for the nature of the list/group, and what is or is not acceptable to the list/group.
If posting to Newsgroups, be aware that many are archived, and the archives are available for a very long time. Don't say anything that might come back and haunt you years down the track. It is generally not possible to retrieve messages once they have been sent.
Be very careful about advertising, some groups welcome it, most others do not!
Watch for and read any FAQs (Frequently Asked Questions) before posting a question to the group.
Messages should be concise and relevant to the group. Don't post messages to inappropriate newsgroups.
Spelling mistakes tend to be the rule rather than the exception. Questioning someone else's spelling is not good netiquette, and not necessary unless it occurs in a critical place, like a possibly mis-spelt URL
Don't get involved in or respond to Flame Wars.
If you find a newsgroup or topic offensive, avoid it. Sending harassing messages to the group is not only frowned on, it may be illegal.
Keep private messages private, don't post to the group as well.

Listen to a channel first, to get the feel of what is and is not acceptable. Above all, respect the culture of the group.
Remember the world is a big place full of very different people.
If you find a topic that offends you, then don't join it.
If you find yourself in a channel that becomes offensive to you, leave it.
Unacceptable behaviour on your part may get you banned from that server.

The complete official memo is RFC: 1855, and is readilly available from numerous sites on the Internet.

An interesting essay on Business Netiquette and additional links can be found at BUSINESS NETIQUETTE INTERNATIONAL.