I don't believe in God. I am either atheist or agnostic, depending on how you want to define your terms
There is one serious problem with arguing this position, which is the misunderstanding of where the burden of proof lies. StevenC.'s posts at the top of this thread illustrated. To say "I don't believe x exists" is to state that you hold the default position. Some object, for which there is no sound evidence either way, probably doesn't exist.
So I, the atheist, must knock down the evidence for the existence of god to nil, and then it is reduced to Russell's teapot: "If I were to suggest that between the Earth and Mars there is a china teapot revolving about the sun in an elliptical orbit, nobody would be able to disprove my assertion provided I were careful to add that the teapot is too small to be revealed even by our most powerful telescopes. But if I were to go on to say that, since my assertion cannot be disproved, it is an intolerable presumption on the part of human reason to doubt it, I should rightly be thought to be talking nonsense. If, however, the existence of such a teapot were affirmed in ancient books, taught as the sacred truth every Sunday, and instilled into the minds of children at school, hesitation to believe in its existence would become a mark of eccentricity and entitle the doubter to the attentions of the psychiatrist in an enlightened age or of the Inquisitor in an earlier time."
It is accepted now that the evidence for the existence of god cannot prove his existence beyond any doubt. The only argument that claims as much is the ontological argument, which fails quite catastrophically, for a number of reasons. We are left to three major arguments: the teleological, the cosmological and the bollocks.
Normally, "the bollocks argument" doesn't make it into text books, but it definitely exists. I use this term for any argument that relies upon something that "seems to me to be the case". I use this term for such things as the moral argument, arguments from the existence of beauty etc. because there is firstly no reason why anyone should accept the fact that the concept being argued from exists, and for those who accept there existence, there is no reason why they can't be some human construct.
Another "bollocks argument" is the "Is this all there is?" line. Yes, it is, there's nothing wrong with that. You might think our explanation of the motion of planets is unsatisfactory "Are these great bodies held in their paths just by one force? Is that all there is?". Yes, why shouldn't it be.
The cosmological argument creates a "god of the gaps", however much you deny the fact. It also relies upon assumptions of causality and/or conservation laws about which we have no right to make assertions beyond the areas we have experience in. This argument normally gets used as part of a series of arguments as I shall come back to.
The teleological argument might well count as a bollocks argument, but it is sufficiently well proliferated for me to want to deal with it separately. Paley's watch doesn't need dealing with, I don't feel, because it has been dealt sufficient a blow by evolution that the man who accepts it isn't worthy of the discussion, in my view. The arguments were reformed after Darwin, but have a tendency to be beaten back to the edge of scientific knowledge, so that now they sit at the fundamental constants being as they are. This argument would be refuted simply by statistical knowledge. You cannot do statistics after the event, looking at only one case. You cannot request that we assert the existence of god because things are the way they are. Of course things are the way they are! The probability is 1, for that.
So normally, we end up with Swinburne's argument, namely that the simplest explanation to everything is that God exists. In science, we go for the simplest argument that works- we proposed Neptune, a single planet, rather than two planets, to explain the motion of Uranus, after all. We were justified in so doing. However, we do that because it explains something. We postulate an example of something we understand- a planet, because that we we don't need a new phenomena we don't. God is a totally new idea, and doesn't explain anything, it just invites the same questions again about god, whilst opening several cans of worms about why isn't the world different if there is somebody designing stuff. (Why the heck did he have to develop a parasite that eats through eyeballs, propogates itself, and dies?). This idea of simplicity is based on the idea that because you only need 1 god, but might need many of some other thing to offer an explanation of phenomena, 1 god is simpler. Suppose you saw a sanddune: what holds it up underneath, more sand, millions and millions of grains of it, or one giant marble?