This is a list of best practices for doing HTML email marketing. It is focused mainly on overall strategy; there are separate articles on email content best practices and email design/layout best practices. If you're the guy actually building the emails, you'll also want to look at Creating HTML Email - Tips & Tricks.
This information is current as of April 2010.
One of the great things about email, and the Internet in general, is that you can test extensively. With email, A/B testing is easy, particularly if you have a large list.
There are several different types of email you can send.
If you're doing email marketing, it is absolutely critical to make sure you actually have permission to email your users.
Here are some excellent articles on email creation that I've found: