Arkham Horror plays quite well. It can play from 1- 8 players, though works best I'd say around 4 to 5. More and it gets too slow (might be my game group though - we have a couple with AP). It's a pure co-operative game where you play on a board of Arkham (the city), moving your investigators across the board to find clues, close gates and fight monsters. It's very Cthulhu like, which means your investigators can go insane and/or die.
You can play the entire game quite well with Arkham Horror the base game without a problem. The expansions do add to it, various debates about which one's you should get. I'd seriously just buy the base game first and then see how much you enjoy it before you expand. I doubt you'd dislike it but...
Of the expansions, I'd say Dunwich Horror (adds a new board) and then King In Yellow and Black Goat are the best.
Just to chime in with a dissenting opinion, it's worth mentioning that some folks find ARKHAM HORROR to be very, very cumbersome. It falls into that category for me of "Games I really wanted to like but just couldn't." There are tons of decks of different cards, tons of rules, and tons of maintenance and upkeep throughout the game. It's an amazing game, don't get me wrong. But playing it is more like work than fun. I say this as a Cthulhu/Lovecraft fan, a board game fan, and a die-hard horror fan.
Oh, and BTW, for what it's worth, ARKHAM HORROR has little in common with Lovecraft's Cthulhu Mythos in terms of theme. Lovecraft's stories are about ordinary fools who go insane because of the evil they glimpse; ARKHAM HORROR is about a group of investigators who go out killing monsters and taking monster trophies.
Again, I think it's a great game, but you have to really, really want to play it in order for it to be a good experience. If that makes sense.
Oh, and all that is without expansions. Once you start adding expansions, ye gods, it's like playing GURPS with all the Advanced combat rules and detailed terrain miniatures to go with it. But if that's what you're looking for, then God bless ya. But many folks want something less complex in a board game.
This may be helpful to anyone considering ARKHAM HORROR:
[ame]http://www.youtube.com/watch?v=8t0urRy-Kbs[/ame]
As you would expect of anyone who makes a video of themselves reviewing a board game, this guy is painfully geeky. But it's also quite informative, methinks.