I am a Nontheist....
Within the philosophy of religion, the designation of nontheist has become much more popular. There are a number of well-known nontheistic philosophers who have been active during the twentith century--probably now more than ever in the history of philosophy. It goes without saying that there is much more non-belief in philosophical circles than in the general public. It would be impossible to list every nontheistic philosopher of the twentith century--which would probably be more than half of them--but there are a number of names which seem to stand out as being excellent nontheistic philosophers:
Tim Crane, David Papineau, David Chalmers, Daniel Dennett, Anthony Flew*, Wallace Matson, Anthony Kenny, Bertrand Russell, Jerry Fodor, Nicholas Everitt, Andrea M. Weisberger, Robert C. Solomon, Julian Baggini, Daniel Harbour, W. V. O. Quine, Walter Sinnott-Armstrong, Paul Edwards, Michael Martin, Robin LePoidevin, J. L. Mackie, John Searle, Thomas Nagel, Richard Rorty, J. J. C. Smart, Theodore Drange, Quentin Smith, Theodore Schick Jr., J. C. A. Gaskin, David O'Connor, Keith Parsons, Jaegwon Kim, William Rowe, James Rachels, J. D. Trout, Donald Davidson, Paul M. Churchland, Peter Singer, Kai Neilsen, Jean-Paul Sartre, Ernst Nagel, Colin McGinn, Michael Scriven, Owen Flanagen, Bruce Russell, John Perry, Paul Kurtz, Graham Oppy, J. L. Pollock, Gilbert Ryle, Robert Nozick, David M. Armstrong, A. J. Ayer, Jan Narveson, Andrew Melnyk, A. C. MacIntyre, Norwood R. Hanson, John Dewey, Patrick Nowell-Smith, Matt McCormick, Richard Gale, Paul Draper, Wilfred Sellars, Howard J. Sobel, Elliott Sober, David M. Rosenthal, Jeffery Polland, John Heil, Anthony O'Hear, H. J. McCloskey, Patricia Churchland, Corliss Lamont, Evan Fales, Ted Honderich, Kurt Baier, Michael Tooley, Ted A. Warfield, Martin Heidegger, Panayot Butchravor, Adolf Grunbaum, C. D. Broad, Ned Block, Philip Kitcher, Douglas Kruger, Terence Penelhum, Corey Washington, Paul K. Moser, Peter Angeles, Richard LaCroix, Walter Kaufman, Sidney Hook, Erich Fromm, Valerii A. Kuvakin, and J. L. Schellenberg.
The most important nontheistic philosopher, in my humble opinion, is still Hume. Many of these philosophers are profound and insighful--as many theistic philosophers are. However, there is no substitute for the "classical insights" of Hume's A Treatise On Human Nature and his Dialogues Concerning Natural Religion. I have always been a big fan of Hume--even when I am in disagreement with him--because of his brilliance, and consistency. Hume's work has many contemporary implications, concerning not only religion but also science. For example, his critique of miracles has application in the paranormal debate.
I am a non-theist, and will probably always remain such. Alvin Plantinga and Richard Swinburne are insightful and brilliant. However, it still seems that the theistic enterprise lacks something. For all of the complixity of the theistic worldview, and the arguments to defend it, there are still significant assumptions it must make to get to its conclusion. The presumption of atheism is powerful, the problem of evil is still a problem, there are many problems with the theistic hypothesis--most notably in the evidence for theism, and the cohorence of theism--and lastly the naturalistic hypothesis is much simpler and seems to be well confirmed.
It appears that Nietzsche was correct: God does appear to be dead.
*Some readers will object that Anthony Flew has recently converted. This was initially true (Dec. 2004), however Flew took back his assertion that a God was needed to explain the scientific evidence saying, "I now realize I made a fool of myself..." He also stated in a letter to the Internet Infidels that his new modest defection from unbelief is a "more radical form of unbelief." (Source: http://www.secweb.org/asset.asp?AssetID=369)