I think this is a great question although I'm sure there'll be 'flame wars' in response if some people notice the thread. lol
From my perspective what I believe would have to be defined before I can tell you why I believe it, so here goes.
I think I believe in a God...at least, I've been brought up to believe in one...somewhere or other he lurks, but it's possible that he/she doesn't exist, in which case I'm fine with that too. I do believe that the Budhist philosophy works pretty well in that I can see how there are lessons to learn in a lifetime and there are different people at different stages learning different lessons in life and I've often wondered why some people have such hard lives and other seem to have it much easier. So with that in mind, I think it's possible the Budhists have got it at least correct in part. Also, I find their belief in the 7 planes of existance fairly comforting since this one we're on now is apparently the lowest which means it can't get any worse.
I think I believe these things because I don't want to accept that maybe when I die I just no longer exist.
Also, since I do believe in ghosts and souls because I think there's overwhelming evidence to suggest they do exist - in my opinion - that there must be something that comes after this life.
It's just what comes next that I'm confused about.
Oh, and I don't believe that God watches every little sin we commit and holds us up for judgement after death. If that were the case, God wouldn't have given us the ability to sin. I believe that if there is a God who sees all, that he knows when we're doing the best we can do, and that if that's what we're doing, then we're square with the keeper. To me, God is more like my conscience, so if I can sleep at night, then what happens next is beyond my control.