What are the names of some reputable health care providers in NYC that
offer affordable ($300 - $600 / month) healthcare benefits to
individuals who are not part of a group plan (such as freelancers)?

It seems that the one you hear about most often is Working Today's
Freelancer's Union (http://www.workingtoday.org/productsservices/products.php
) but I don't know what their rates are like.

Also, even if you're a freelancer, you might be eligible to join some
sort of group (local or otherwise) that offers some sort of health
insurance access.  For instance, if you're a Graphic Artist, I think
the American Institute of Graphic Artists affords some healthcare
options to its members. Similarly, if you're an author, the Authors
Guild may have some healthcare options.  They might not be
inexpensive, but they're certainly less expensive than buying similar
insurance without such a group affiliation.