24 Hour Fitness isn't exactly cheap, but it's a good deal compared to other Manhattan gyms. For a $29.99 initiation fee (waived if you're a student), you can join for $64.99 a month ($69.99 if you want access to all 24 Hour fitness clubs). You have to pay the first and last month upfront (which isn't refunded if you cancel), but there's no commitment beyond that and you can cancel your monthly contract at any time.
Why is it a good deal? The Crosby Street location is brand new and boasts lots of space (three floors), new equipment, a lovely class studio that can hold up to 75 people, and, for spin enthusiasts, a large, well-ventilated spin room with shiny, new bikes. They also have such high-tech features as cardless entry (they scan your index finger instead) and lockers that use codes that you key in with each use--no more carrying your own lock. And it's a great location, to boot.
http://www.24hourfitness.com/FindClubDetailGeneric.do?clubid=00726
