I recently got the Chalmers volume dealing with the foundations of ontology. I've been interested in this question for the last few months, and here are some of my thoughts on the matter.
Most philosophers these days accept the Quinean view that one is committed to whatever one quantifies over. If I say there are colors, i.e. (Ex)(x is a color), and I think my saying is true, then I am committed to the existence of colors, end of story. Most contemporary debates as in the Chalmers volume are between those who hold to this Quinean view and a few people who want to revive some of the distinctions of Carnap.
From an Aristotelian-Thomistic perspective, the dominant Quinean view is incorrect for a few reasons. First of all it is wrong to say that metaphysics is just about finding what there is or listing an ontology. As this volume shows, many if not most philosophers these days accept this definition. Rather, ontology is just one part of metaphysics, the ultimate purpose of metaphysics being, in the spirit of Aristotle, the study of being qua being, i.e. the study of the fundamental nature of reality. This involves not only ontology, but also finding the essences of things and finding the various relations of ontological dependence these things stand in to each other.
Also, I think there is some confusion in the modern Quinean conception of existence, where being quantified over is taken to be being simpliciter (in Thomistic terms). On the contrary, there is being simpliciter, then the various diminished senses of being (being secundum quid, in Thomistic terms), as for example the being privations have or being in potentiality. Many of the things we quantify over might have being in one of these imperfect and diminished senses, but just because we quantify over them doesn't mean we have to conclude they have being simpliciter. This makes ontological questions quite trivial. For instance, of course there are numbers and numbers have being; one need only observe the fact that 2 is a number. The real question is finding out whether they have being simpliciter or being secundum quid, what categories of being they fall under, and in what relations of ontological dependence do they stand? Interestingly, in this volume, Kit Fine and Jonathan Schaffer seem to come to similar conclusions, though I don't think Fine is rigorous enough to be convincing to other philosophers already steeped in Quineanism. I'd like to write a paper on this at some point so as to make the Aristotelian position a bit more clear than Fine does.
