Raymond Chen talks about using very long e-mail signatures, and on the most part I agree with him.
However, he makes one complaint that I must object against. In particular, he says "we already know your name and email address since it's in the message header". Yes, you do, but isn't it common courtesy to sign a message with your name?
The addressee knows his/her own name too, but I still think it's a good idea to start with "Hello X" or "Dear Y", or "Dear sir/madam" if I'm not sure.
Personally I think most people treat e-mail very loosely, at least in the Netherlands. If I send e-mail to a person or company that I don't know, I will treat the e-mail as if it was a letter. I will open with "Dear sir/madam" etc. But invariably I get a message back starting with "Hi Sven". Now I don't mind that sort of thing, but it's a strange assumption to make. Certainly if it was a letter people wouldn't automatically assume they could be so informal.
The Japanese live at the other end of the spectrum. Using first names is a big social faux-pas anyway over here (especially fun since no one here can pronounce my last name), except among very close friends. But many Japanese break Raymond's rules even more drastically by telling me my name, their name, the message, and then their name again.
Typically the messages will end up looking something like this:
Y here from the office of Z.
Message goes here.
Well, at least there's no doubt over who sent the message. :)