The categories of: atheists; agnostics; and theists apply to scientists, as well as everyone else. As JesseM has already documented, the list of atheistic scientists is very long and includes such luminaries as Steven Weinberg, Stephen Hawking, and Richard Dawkins. AikenDrum has already talked about Weinberg.
I know of no specific examples of well-known scientists who are agnostic, but I'm sure there must be quite a few. The agnostic category can be split into at least 2 subcategories: scientists who haven't taken the time to think deeply about such matters; scientists who, after thinking deeply, have come to the conclusion that not enough information exits to form a conclusion.
A more detailed account of Weinberg's (atheistic/agnostic?) views can be found in the chapter What about God? from his book Dreams of a Final Theory: The Search for the Fundamental Laws of Nature. In this chapter, after giving a few specific (but unnamed) examples of devoutly religious physicists, Weinberg writes "But, as far as I can tell from my own observations, most physicists today are not sufficiently interested in religion even to qualify as practicing atheists."
In spite of this, there are examples of first-rate physicists who are theists. These include Paul Davies (?), Chris Isham (a leading expert in the conceptual problems associated with quantizing gravity), G. F. R. Ellis (of Hawking and Ellis fame), John Polkinghorne (a former head of the particle physics group at Cambridge turned Anglican priest), and Nobel laureate Abdus Salam (of the Weinberg-Salam model).
Theists also can be divided into 2 groups: theists that believe that God got the ball rolling and then stepped back to admire his handiwork; theists that believe in divine intervention.
Ellis, Polkinghorne and Salam fall into the second group. In order to avoid miracles, as defined by David Hume, as much as possible, Polkinghorne and Ellis have both written speculatively about how this might be accomplished without noticeable violations of the laws of physics. I thing that many physicists will find their ideas to be quite repugnant.
Chris Isham's views are given http://www.metanexus.net/archives/printerfriendly.asp?ARCHIVEID=7500 .
Those interested can listen to Paul Davies, author Kitty Ferguson, John Polkinghorne, and Steven Weinberg discuss their views on the relationship between science and religion. This
discussion aired for the first 17 minutes of the CBC Radio science program Quirks and Quarks on Dec. 21, 1996.
On this program, Davies seemed to adopt a somewhat more cautious tone than he did in his book The Mind of God (fascinating reading even for atheistic physicists), which he ends with "Through conscious beings the universe has generated self-awareness. This can be no trivial detail, no minor by-product of mindless, purposeless forces. We are truly meant to be here." In the radio program, Davies does say "Weinberg and I look at the same set of physical laws, and we draw our own conclusions. He looks at those laws and he waxes lyrical about their beauty and harmony and indeed their ingenuity, and but comes to the conclusion that at the end of the day it is all ultimately absurd. I would say that I look at that set of laws and for me it is overwhelmingly suggestive that there is a point to it. But, you know, science can never prove or disprove the existence of something which is beyond science ... All it can do is give circumstantial evidence ..."
Regards,
George