Here are all the Promo Codes that I am aware of - both past and present:
Codes
Definition
34BEV
3rd and 4th guest free Beverage package
34DIN
3rd and 4th guest free and dining credit
34DINE*
3rd and 4th guest free and dining credit
34KDF99*
3rd and 4th guest for $99 USD
34PHO*
3rd & 4th Guest Free & Photo Package
34SHORE
3rd
Since many of you have provided new codes, I am posting an updated list, although a few we don't know the definition.
Special thanks to sjacobs11 for the many additions.
Codes
Definition
1DINPK
1 Meal Specialty Dining Package
10OFFS
Specialty dining for two, wine trio, bottled water and chocolate truffles
34BEV
3rd and 4th guest free Beverage package
34CHO
3rd and 4th guest