If you are associated with or can associate yourself with a convention, they offer 1, 2, and 3 day park hoppers. In June 03, we were able to walk into World of Disney on International Drive and purchase an extra ticket.
Here's a link for a May convention- ticket orders due by 4/15/04 and tickets must be used by 12/31/04. https://secure.hes-services.com/WDWticket/LRP.asp
unlike park hoppers this tickets will expire.