Seriously, assuming that they have a polyurethane (sp?) finish, from everything I've heard the best thing is water--doesn't get much more natural than that. The idea is to use a well-wrung out rag, since any water sitting on floors is bad for the finish.
If you're worried that that's not doing it for the dirt, I'd put just a little bit of a fairly mild soap, maybe even dish soap (for handwashing dishes, that is) in a bucket of water and do the damp rag treatment.
I have also heard that vinegar, which is part of many natural home cleaning recipes, isn't the best thing for the finish, for what that's worth.
This really makes it sound as if I actually clean my floors, doesn't it?