Can atheists be spiritual?
The problem with answering whether atheists are spiritual or not is that the term "spiritual" is so vague and ill-defined most of the time. Usually when people use it they mean something similar to, but nevertheless very distinct from, religion. This is probably an improper usage because there are very good reasons to think that spirituality is more a type of religion than anything else.
So what does this mean when it comes to whether atheists can be spiritual or not? If the general usage is mistaken and spirituality really is best described as a highly personalized and privatized religious belief system, then the answer to the question is clearly "yes." Atheism is not only compatible with the adoption of a public, organized religious belief system, it is also compatible with the adoption of a very personal and private religious faith.
On the other hand, if spirituality is treated as "something else," something fundamentally different from religion, then the question becomes harder to answer. Spirituality seems to be one of those words which has as many definitions as it does people trying to define it. Often it is used in conjunction with theism because people's spirituality is "God-centered." In such cases, it is unlikely that you could find an atheist who is "spiritual" because there is a real contradiction between living a "God-centered" life while not believing in the existence of any gods.
This is not, however, the only way the concept of "spirituality" can be used. For some people, it involves a variety of very personal things like self-realization, philosophical searching, etc. For many others, it is something like a very deep and strong emotional reaction to "wonders" of life — for example, gazing out at the universe on a clear night, seeing a newborn child, etc.
All of these and similar senses of "spirituality" are entirely compatible with atheism. There is nothing about atheism which prevents a person from have such experiences or quests. Indeed, for many atheists their atheism is a direct result of such philosophical searching and religious questioning — thus, one might argue that their atheism is an integral component of their "spirituality" and their ongoing search for meaning in life.
In the end, all of this vagueness prevents the concept of spirituality from carrying a great deal of cognitive content. It does, however, carry emotional content — much of what people describe as "spirituality" seems to have much more to do with emotional than intellectual reactions to events and experiences. So, when a person is using the term, they are more likely trying to convey something about their emotions and their emotional reactions to things than a coherent set of beliefs and ideas.
If an atheist is wondering if it would be appropriate to use the term "spiritual" when describing themselves and their attitudes, the question that must be asked is: does it have any emotional resonance with you? Does it "feel" like it conveys some aspect of your emotional life? If so, then it may be a term you can use and it will mean just what you "feel" it conveys. On the other hand, if it just feels empty and unnecessary, then you won't be using it because it just doesn't mean anything for you.