Well I'm going to assume you're leaving RPG's and Minatures off the table so I'll give you my favourite non traditional ones  (For example: love Risk but that's a triditional one) in no particular order:

Settlers of Catan
Tannhauser
Talisman
Arkham Horror
Betrayal at the House on the Hill
Supremacy
Rail Baron
And any of Mayfairs Rail games: Euro Rails, Nipon Rails, Iron Dragon... etc. 

I'm lumping the rail games as one because they all have pretty much the exact same rules and game play, all that changes is the map and contracts in the game.

Aestolia's list +

Pandemic

Powergrid

Dominion

Puerto Rico

ooh powergrid.

Also a good one (though not a board game) is Fluxx. It's a card game (not collectable).

Fluxx is fun, and is made by Looney Labs, which is a small game company that has a pretty good run when it comes to card games.  They've also made Chrononauts, which is good for a group game night, not to mention Back to the Future the card game, whose rules are based on Chrononauts.

Plus, Battlestar Galactica the board game is super fun. 

You guys can also include card games like Fluxx

I've recently found in my playing experience (besides games that have already been mentioned); these are some of most fun:

Dungeon Lords

Agricola

Descent

The Last Night on Earth is decent, but flawed in some of its design choices.