I just added the following checklist to the Email Marketer's Club Wiki. It contains the things you should check before you send out your email campaigns. I'm sure I've missed stuff, so feel free to help build a thorough checklist here.

The list

• Are you sending it to the correct list? Particularly important if you're an agency sending on behalf of multiple clients. Or, if you just want to send to your test list, make sure that it's selected here (and not your "real" list).
• Do you have permission to send emails to the people on your list?

• Does your brand appear in the “from” line?
• Is the subject benefit-oriented instead of "selling"? Make the first 45 characters or so count. Ask yourself, "What will make a reader immediately open this message?" Rewrite the subject line at least 10 to 20 different ways to come up with the best approach. Test subject lines.
• Are you sending the email at a time when the recipient is most likely to read it?

• Did you includes a link to view the email online?
• Are you featuring your brand or logo prominently?
• Does the email include the newsletter title or strong headline?

• Is it personalized with the recipient’s name? Do all the merge fields work properly?
• Did you include an opening paragraph that pulls readers in?
• Does the copy read like it comes from a person?
• Does the email includes benefit-oriented information that is also engaging?
• Are you making it clear to the reader what you want him to do? Make the call-to-action link prominent, not only on top of the message, but in several additional places in the email as well.
• Do you have multiple calls to action? Both as text links and images? 
• Can the email be easily skimmed? Did you use short paragraphs and bullet points?
• Is it a manageable length to read online?
• Did you not include too many topics in the email? Maybe it would be better to split the content over two emails?
• Did you check the copy one last time for spelling mistakes?

• Are you using images sparingly? (only when they advance the goals of the email)
• Are your all your images loading and do they load quickly?
• Do all the links work? Don't forget to check the links in the text version!
• Are all the images linked?
• Did you check what the email looks like in different email clients such as Gmail, Yahoo, Hotmail, Outlook, Windows Live Mail, Thunderbird, Entourage, Lotus Notes...
• Are you using the correct email template for the campaign?
• Did you remember to create a plain-text version of the email?

• Do you have a working unsubscribe link?
• Did you include your physical (USPS) address?
• Are you protecting your content with a copyright notice?

• Is the landing page live?
• Is the content and the design on the landing page consistent?
• Is the content not too long?
• Is the call to action obvious?
• Is the landing page copy not too long?
• Is the registration form not too long? Ask only for the basic information you need. Long forms have a higher exit rate. You can always ask more questions later.

• Are you asking recipient to whitelist the “from” address so future e-mails get delivered to their inbox?
• Are you including a viral call to action, encouraging the reader to share your email with friends or colleagues?
• Did you include a subscription mechanism for people it is forwarded to?
• Are you sending the email in multi-part MIME format?
• Did you incorporate tracking and reporting?

Do you think something's missing? Add it to the checklist here.