First off, I’ve got to throw in my lot with the Backgammon players.
That being said…
If you have 4+ players who want to kill about 10-12 hours, try getting a copy of Civilization (by Avalon Hill). The game is out of print, but there is usually a copy or three available on Ebay at any given time.
A good game for detail-noticers is Scrutineyes. This game has 12 pictures from Mike Wilkes’ book The Ultimate Alphabet (a great book, BTW). These pictures are crammed with things that all begin with a certain letter of the alphabet. The object is to find as many objects within two minutes that begin with the letter for the picture. After you and all your opponenets reveal your answers, you search again for another minute and try to find new things (and you are penalized if you mention something mentioned in the previous round).
A great card game for 4+ people is The Great Dalmuti, a card game where the people at the bottom of the pecking order have to crawl and claw their way to the top.
A good card game for two players (but better with more than two) is Guillotine. Set during the French Revolution, you and your opponent manipulate a line of convicted criminals in hopes of bagging the more valuable heads.
An interesting board game (if you can find it) is Robo Rally where you have to have your robot navigate a factory filled with all sorts of hazards.
If you want simple strategy, try Cathedral. In this game, you and your opponent (only two players) place buildings on a grid in an attempt to control more land.
Another simple strategy game (already mentioned above) is Twixt. The very simple premise of this game is to build a line from one end of the board to the other before your opponent does the same the opposite way.
A great strategy game, mentioned above, is Pente. The premise of this game is simply to get five stones in a row. Of course, you also need to watch out, because your opponent can steal your stones two at a time (and if he gets 10 of them, you lose!).
Then, of course, you have the classics. Scrabble (a personal favorite), Monopoly and Trivial Pursuit are always good for a fun play.
Lastly, find a book that lists different variations on Poker. If you play with enough variations, you can keep going for quite a while before boredom sets in.
Zev Steinhardt