Pascal's Wager was an idea proposed by Blaise Pascal that, since God's existence is not likely to be determined, one should live as if God does exist as there is nothing to lose but everything to gain.
The idea goes something like this:
If you believe in God and God does exist you receive the greatest reward. (+)
If you believe in God and God does not exist, nothing happens. (Neutral)
If you don't believe in God and God does exist, eternal hell. (-)
If you don't believe in God and God does not exist, nothing happens. (Neutral)
There appears to be no "negative" in actual belief in a God - but that's not the whole story.
The "wager" assumes that you are worshipping the correct God - what if you have chosen to worship the wrong one (considering there are numerous major religions, you have, at best, a 20% chance of worshipping the right one)? Wouldn't a God be more upset that you've been worshipping the wrong God rather than no God at all?
Wouldn't a God worth worshipping know that you were only hedging your bets, too?
Couldn't you be a "better" person if you spent your time helping others and not wasted it worshipping a God?
The wager also (falsely) assumes (and implies by description) that God's existence versus his non-existence is 50/50.
Would a God who "created" humans with the ability to think and question punish someone for doing just that?
Update: Check out John Loftus' "Pascal's Wager Revisited" - It's much more in depth. (My apologies for "inventing" an already used title.)