A friend told me about freetoursbyfoot.com , a company that offers just that, free tours.
Run by some of the best guides, I have come across, they offer the tours for free, you just pay the guide at the end what you think the tour was worth.
So far, I have done two of their tours:
Brooklyn Graffiti and Street Art Tour
Central Park: Lower Section
Each time, I have paid my guide a $25USD tip. Each tour has been at least 2 hours long and provides a great insight into some of the great things to see in NYC.
Free Tours by Foot offer tours in a number of different cities. You can book on their calendar online and you receive an email confirmation. The day before, your guide contacts you by email and gives you more information as well as contact details.
It is a simple process and best of all- free to reserve your spot.
I now recommend this company to my clients in addition to the New York Pass and $31, 7 day subway pass.