The Internet is no different, as unwritten rules have been made concerning everything from e-mail signatures to replies. Don't make your email signatures longer than four lines. Also, delete their signatures if one is present. If you receive a multiple recipient mailing and you reply to all, you'll have plenty of confused and possibly irate people.
Chain letters are a no-no.
Gullibility is also an issue. Don 't believe the virus mailings or the charitable donation mailings. 99,999 to 1, they're fake. While it's definitely ok to post and to offer differing viewpoints, it's not ok to insult others. Use the web responsibly.