My colleague Colin Allen is a representative of a consistently naturalistic point of view.
Are you looking for people with a specifically atheist response to Plantinga's arguments in particular? If so, which of AP's arguments?
The more specific you can be with your question, the easier it will be to give you a useful answer.
Well, according to Wikipedia, Michael Martin is such a philosopher. Also, the ontological argument in every form has been pretty thoroughly eviscerated by any number of philosophers--atheist and otherwise. I wouldn't restrict myself to just atheists when looking for well reasoned arguments against Plantinga.