When searching for such material, it really strikes you how much less there is of it than the Christian equivalent. That doesn't necessarily reflect badly on us, as evangelicals really go overboard in this department. I walked through the National Mall in Washington DC this July 4th and could have pocketed about half a dozen different pamphlets if I'd wanted. (In case you're curious, half were from Jews for Jesus.)
Here is a start: various print media from the Freedom from Religion Foundation. Some of it is US-specific and it's all very wordy, but if you can fit it on a pamphlet it might be useful.
Other than that, pickings are slim.
Perhaps our viewpoint simply comes across better in books than on scraps of paper. If you want something you can whip out the way a Christian would their pocket New Testament, I recommend the compact anthologyThe Portable Atheist, compiled by Christopher Hitchens.
- SmartLX
Edit: At last a resourceful atheist has taken it upon himself to supply us with a handy toolbox of viable paraphernalia. Cheers, Cyberguy.