Think before you click. Is your e-mail necessary? A 'crazy' office joke is non-essential; latest sales figures are. Avoid an unspecific 'Hi' subject line and don't waffle about weekend escapades. Be concise and clear about action required from the recipient. Re-read your note before you hit 'Send'.
Judge the tone. It's okay to greet mates with 'Yo!', but not the CEO or someone you don't know well. Irony and humour don't travel well, and never send an e-mail in anger. 'Shouting' in capitals will make you seem unhinged.
To cc? Don't copy in everyone from the postie to the chairman in an attempt to cover yourself. Be selective: sending a company-wide note announcing your birthday is a bad idea.
Be discreet. Keep confidential information just that, and don't write anything you wouldn't put in a letter. No office gossip or porn.
Sent in error? Contact the recipient immediately to apologise and explain the cock-up. If your error has high embarrassment potential, tell your boss before he/she receives a complaint.
To reply or not? Don't rush to answer. Acknowledge an e-mail that needs a considered response, and return to it when you can. Don't 'Reply to all' unless you really mean to.
Attachments? Attach files only if they are essential (your skiing snaps aren't). If you can, send a web link rather than a 5Mb Word document.