Scads of off-Broadway and off-off Broadway houses use volunteer ushers who in exchange for working for an hour or so (usually before curtain time), get to see the show for free. Most Broadway theatres have to use ushers from a union, but New York theatregoers have a myriad of other opportunities off the Great White Way to see theatre. Theatres like Second Stage, the Roundabout, Playwrights Horizons and the Signature are staging some of the most challenging and entertaining works around. Many of the theatres which use volunteer ushers are not-for-profit, so you are not only getting to see a good show, but are supporting worthy organizations in the process, which Gothamist thinks is a lovely thing.
Heres a place to start for contact info, but by no means is this a comprehensive list. Call around and check theatre websites for additional ideas and get cracking. The season will heat up enormously shortly after Labor Day and shows will get booked up!