Let’s face it, the documentation on the MailChimp site is paltry, at best. I’m going to add things here as I come across them.

First up, full documentation for the DATE merge tag.

Format Options
See the PHP Documentation for all the character codes you can use in the FORMAT string. (Thanks for the tip, Chad!)
*|DATE:l F jS, Y|* = Monday January 15th, 2009
*|DATE:m/d/y|* = 01/05/09
*|DATE:D j M Y H:ia|* = Mon 15 Jan 2009 05:53pm

Please let me know in the comments if you discover anything is not as documented here (or figure out anything that I haven’t!)