Hemant Mehta, aka The Friendly Atheist, is looking for contributors for a variety of topics.
These would be regular contributors, but he's also looking for contributions of single, longer posts. He does have some requirements, but would also provide compensation. See his Submissions page for more details.Book reviews (~1 post/month)Law (~1-2 posts/week)Pop Culture (~1-2 posts/week)Interviews (~1-2 posts/week)Politics (~1-2 posts/week)
Another option would be the Guest Blog Storms of Atheos is currently working on getting going. It wouldn't pay anything, but the criteria for getting something published is much lower. To submit a guest post there, just e-mail email@example.com.
(Credit to @hemantmehta)