This is a tough question to answer, as shared housing with like-minded progressive people is generally found through good old word-of-mouth, which means you would probably have to actually be in NYC to start looking for this kind of arrangement. There are no central resources for cooperative houses here.

We can suggest some spaces that may have ads posted for available housing and are just good places to network through the activities and projects they host.

ABC No Rio
156 Rivington Sts (between Clinton and Suffolk Sts)

Sixth Street Community Center
638 E. 6th St (between Aves B and C)

Bluestockings Books
172 Allen St (between Stanton and Rivington Sts)

Times Up!
49 E. Houston St (between Mott and Mulberry Sts)

All of these spaces are in lower Manhattan, but people from all over the city converge here.

There is also the Park Slope Food Coop in Park Slope, Brooklyn. It's the largest member-owned and -operated food co-op in the country, and there is a bulletin board inside the store with all kinds of housing-related postings, including for shares. There are online classifieds that you can peruse, though more people hang up ads in the Co-op itself than use the Web site.