WordPress: Adding custom boxes to posts

I stumbled on a great post about how to custom boxes for your wordpress posts. In this case it was promo boxes, but it could be used anytime you want to relate certain custom types of content to your wordpress pages (for example, a portfolio of work, a list of employees, etc).

http://wordpress.stackexchange.com/questions/5750/custom-field-meta-populated-by-dropdown-of-existing-posts