HTML e-mail tips
I know, I know … HTML e-mail is evil, and personally I would never subscribe to an HTML newsletter, no matter how interesting the contents. However, if you are going to use HTML e-mail for your newsletters, you might as well do it right (or as least wrong as possible). Mailchimp has got a handy guide on HTML e-mail (PDF, at the bottom under “HTML Email Design Guide”).
This one has moved to its own page: http://www.mailchimp.com/resources/guidebook.phtml.
Nice guide. I’ve also written a guide on how to create and send HTML e-mail: “Tips for creating and sending succesful and compatible HTML e-mail“. It deals with a lot of obstacles you run into, how to avoid them, a lot of tips and tools you can use to create more compatible HTML e-mail.