This is my normal response to christians who post it:
What if the god that really exists sends all good people to heaven and all bad people to hell except people who pretended to know for a fact another god existed? then all good atheists are going to heaven but YOU ARE GOING STRAIGHT TO HELL. Why not repent when you still have time? Why risk eternity in hell?
What if god loves music and sends all musicians to heaven, but all other people to hell? How can you risk eternal torture by not playing at least four hours a day?
And how can you worship a sadistic god that tortures good people - most of the people who ever lived - for all eternity?
Pascal's wager:
"If god exists, it's infinitely better to believe, since you get heaven instead of hell for eternity. If he doesn't, it doesn't matter since you're dead anyway. So overall it's better to believe"
This is, of course, false.
Some of the problems with the argument:
* The assumption that there is an afterlife with a heaven and hell
* The assumption that the god cares about belief in him/her above all else
* The assumption that if you believe in a god, it will definitely be the same god that actually exists.
* The assumption that you lose nothing if it's false. Religious belief costs people plenty - money donated to churches, time spent praying, marriages ending because of religious differences, lives lost because of relying on prayers or refused medical care, wars... need I go on?
* The assumption that people can believe in something simply because it benefits them. Would you believe goblins exist for twenty bucks? Why not?
* The assumption that any god won't see through the "believing just to get into heaven" ploy.
For more:
http://www.abarnett.demon.co.uk/atheism/wager.html
http://plato.stanford.edu/entries/pascal-wager/
http://www.infidels.org/library/modern/theism/wager.html