I just came across a very good list of 101 atheist quotes collected by The Atheist Blogger; there are several gems so check it out. My favorite (among the ones not familiar to me): "The invisible and the non-existent look very much alike." - Delos B. McKown.
There are, I should note, a number of apparent misquotes and misattributions. Mark Twain (d. 1910), for example, surely could not have known the universe was billions of years old and the quote attributed to Hobbes (assuming "Thomas Hobbs" refers to Thomas Hobbes) fails my smell test (and Google knows it not).