(Unsure if this should be in "Regional" or "Tools"- move if you see fit!)
Fractured Atlas has the beta up of a their online rental system. You can specify the date(s), what you need the space for, any amenities (wifi, mirrors, etc) and you can sort results by price and neighborhood. I was playing with this yesterday and found it very helpful.
http://nycpaspaces.org/Bonus: booking some of these venues online earns you an additional 10-20% off.
Moderator note: revised subject line for added clarity since this will be a sticky post. -kmc307