As strange as it may initially seem, there are atheists who believe in ghosts. Since atheism refers to one's lack of belief in gods, there is nothing about the definition of atheism that makes this problematic. While most atheists do seem to be materialists in the sense that they do not believe in anything outside of nature (i.e., anything supernatural), atheism does not require this sort of materialism. So yes, someone can be an atheist and still believe in ghosts, spirits, and the like. But for those of us who arrived at atheism via skepticism, critical thinking, and empiricism, the presence of ghost-believing atheists is a bit odd to say the least.
While playing with StumbleUpon this morning, I happened across an atheist blog written by someone who was trying to reconcile atheism with "spirituality." I have no interest in this and moved on quickly. Moments later I found myself at Atheist Nexus, shaking my head in disbelief over a thread in which someone was asking about others' experiences with "hauntings."
Maybe I need to make more of an effort to understand atheists who believe in ghosts, spirits, monsters, etc. Clearly, the path by which they came to atheism is very different from mine. Perhaps such individuals could teach us something. How about you? Do you believe in ghosts?