Monopoly is a classic, and my all time favourite
Axis and Allies is like Risk but on crack
Sports Franchise is a fantastic game where you build a sporting empire and take down your opponents, you play games, and win a championship with the all mighty dollar determining the winner!
Taboo is a very simplistic game that can last hours or take mere minutes. The simplicity keeps you coming back for more while on your toes and laughing all night long.
Edit: Totally forgot about Settlers of Catan. That is another gem, I'm a huge fan despite only playing it once after being introduced to it at the end of last semester.