Either the lack of belief that there exists a god, or the belief that there exists none. Sometimes thought itself to be more dogmatic than mere agnosticism, although atheists retort that everyone is an atheist about most gods, so they merely advance one step further.

http://www.pluralism.org/resources/tradition/atheism_links.php A list of internet resources on atheism

http://www.philosophytalk.org/shows/existence-god-0 An audio discussion of the arguments for and against God’s existence