Since the last few people have been discussing what they are, I'll join in too.
I now identify as an agnostic (formerly being an atheist, and prior to that being a CofE Christian), solely because there is no definitive evidence to prove or disprove the existence of a God/Ultimate Being.
There's no creationist "science" that influences my view, whereas I believe the theories of Evolution and the Big Bang to be correct and that they are definitely the way in which the universe came about and how life came to be diverse.
But ultimately there's no evidence to say what came before/started the Big Bang, and there's no evidence to say it wasn't God who created it, nor is there evidence to say God did.
Nothing else will really convince me that there is a God. No logical arguments, no claims of religious experience, design or miracles or claims of near death experiences, ghosts or spirits.
It'd have to be concrete empirical proof that there exists a God, or that there doesn't.