Islands Dining Room at Royal Pacific Resort has a great dinner buffet. It has different themed nights so I would check with the hotel. I believe Fridays are Seafood. There's pictures from it on our food pics thread.
As mentioned above, Trattoria at Portofino Bay has their dinner buffet at night as well. But there are no buffets in the parks except for Confisco Grille at breakfast time with characters.
There's also a Kids Buffet at Pastamore in CityWalk for $8.95, while parents order off the menu.