Other board games

[Snowreap]

Settlers of Catan isn't terribly hard to learn; having just the right number of players is probably more of an obstacle than learning the rules.

Blokus is a good board (style) game, but limited to 4 players maximum.

any video game that encourages taking turns works well as a party game, regardless of how many people are present. DDR and Guitar Hero are both good, but if I had to choose only one I'd go with Guitar Hero since you don't need potentially expensive third-party controllers (the 'stock' DDR dance pads are awful).

there are also some fun (non-collectible) card games, marketed as children's games, which might be of interest. for example, Slamwich and SET.

-ken