In Poland, Coca-Cola is online with an advergaming site to celebrate the Christmas period.
The site features a total of six advergames. I managed to play only a Xmas-Coke version of the popular Chinese game Mahjong, then my score wasn't good enough to access the second game. And here comes the best part of the site: only the first advergame is open to everybody, all the others require a minimum score to enter. If games are good and funny to play I really think this is a sticky idea to keep people on the site.
Sorry but my Polish is not good enough :-) to tell you if there is also a competition associated to the site. If any reader can help... thank you!