The Broke Student's Guide to NYC
Surfin' around I've found this link from the New York University:
www.nyu.edu/src/brokestudents/
A bit outdated but still useful, it contains some gems such as:
"If you are really and truly broke and you need to get somewhere…walk! New York is a walking city (although walking is preferable anytime but the winter months!). Get out there and see the city. Walk around town and take it all in."
Worth a look