I have two 8-player games in my collection that I've brought along to my weekly cafe boardgames group. (Well, they have variants or expansions that allow 8 players, anyway...)
A good, solid gamery game. I expect the 8-player version would be too intimidating for people who hadn't played before, but it's definitely the sort of game that people can become obsessed over, and want to play again and again. Individual turns are really quick, and there's a lot of tension in the fact that everyone's role for the turn is secret until it comes around, so there's definitely plenty to hold people's interest while they're waiting for 7 other players to take their go!
Kind of a polar opposite to Citadels, in that I would have no qualms about letting a beginner sit down to the game, but I don't think there's quite as much to hold the long-term interest of dedicated strategy-heads. Bang! is a lot of fun, as players form alliances (or, more often, get embroiled in vendettas) in a Wild West shootout, where no one is quite sure who is on the Sheriff's side and who wants him dead (Werewolf style). The one issue with it as an 8-player game is the same one that Werewolf has: it's possible to be knocked out of the game pretty quick, and if that happens it's tough to be just a spectator for the next half hour or more.
ETA: Here's a list of LOTS of 8-player boardgames for anyone interested in really exploring the possibilities: http://boardgamegeek.com/tag/8-player. Hopefully we have already covered the major contenders between us!