Fortune and Glory is an AWESOME board game, if you've got the stamina for it.. 2 hours is a REALLY short game, 4 hours is easy, 6 hours is doable. I shit you not. Aside from being long, it's gigantic and can be very off-putting for less veteran gamers who will initially find it far more complex than it actually is. The biggest downfall of this game is it's $100 pricetag.
Co-operative or competitive, 1 to 8 players
http://boardgamegeek.com/boardgame/95103/fortune-and-glory-the-cliffhanger-game Forbidden Island.. well it's a game just like my girlfriend.. fast, fun and easy. ;) Unlike my girlfriend, it's also pretty cheap. at around $20.
co-op, 1-4 players.
http://boardgamegeek.com/boardgame/65244/forbidden-island Incursion is pretty epic if you like strategic squad-driven combat. It's got some complexities, but it's pretty simple for a veteran gamer to understand too. Not overly well balanced though I have to say. Can fall pretty much anywhere in the time to play scale. Middling pricerange at about $60 new, there's an expansion for it as well, which adds some new gameplay options.
Competitive, 2 players. (SNAFU adds co-op and makes it IIRC 1-3 players)
http://boardgamegeek.com/boardgame/40603/incursion Settlers of Catan is a widely popular economic strategy game. Pretty easy to pick up, not overly off-putting to newbie gamers. Middle of the road length of game, and fairly inexpensive at $35 or so. There's also lots of expansions for it.
Competitive, 2-4 players (expansions can add more).
http://boardgamegeek.com/boardgame/13/the-settlers-of-catan While technically a card game, Poo is great fun. It's a good travel game. It's highly immature, you're monkey flinging poo at eachother!, super easy to play, pretty quick to play- it looks like it's a little pricey these days though? Maybe not being produced anymore.
Competitive, 2+ players.. the more the better.
http://boardgamegeek.com/boardgame/60579/poo-the-card-game And also a card game, I would be remiss if I did not mention Munchkin.
If you enjoy goofshit humor, making fun of genre stereotypes, and having a giant pile of insane loot.. Munchkin is the game for you. It's easy to play, anyone can pick it up. Games can be very fast or very long, depending on players and playstyle.. and it's sure to create a lot of laughs and mayhem. While the base game runs about $20, there are more expansions and versions than you can shake a stick at, easily allowing you to spend several hundred on it if you want.
Competetive, 2+ players (3-6 is best)
http://boardgamegeek.com/boardgame/1927/munchkin