The one guy i saw with a knife here had a butterfly knife... and when I asked about it (he was using it to open a box in the store he was working in) he just stated that he didn't know any laws about it but you could get it in chinatown... That didn't sound to me like a very intelligent thing to do in a city where (i've heard) officers routinly stop individuals so that they can
perform 'terry frisks' on them.
I'll give you a brief overview of NYC knife laws:
Prohibitted
* carrying knives with blades longer than equal to or greater than 4" (including the unsharpened portion)
* gravity knives
* switchblades
* carrying knives in a visible manner
* "Dangerous Knives" (NYS Penal Code term)
* daggers
NYS State law defines a gravity knife as a lockblade knife that can be opened by centrifrugial force. In short, that means that if the blade locks, and it can be flicked open (either by holding the handle or blade), it is a gravity knife. Carrying a gravity knife constitutes "Criminal Posession of a Weapon" , which depending on your prior record could be charged as either an A Misdomeanor or a D (IIRC) felony.
NYPD rabidly persues weapons violations, especially when it comes to carrying knives. In most other counties they aren't as emphatic about prosecuting people for gravity knives, but people have been charged under the same interpretation. There is an exemption in NYS law that allows the carrying of a gravity knife or switchblade while engaged in licensed hunting or fishing activities. The sale of gravity knives (and switchblades) is illegal in NYS, the exemption was really meant for people who already owned them. However, there is many retailers who do sell illegal knives, and are not shut down by NYC or NYS officials.
The other issue major issue that you will face in New York City is that NYC Administrative Code prohibits visibly carrying a knife when not in actual use (ie cutting something) with a couple of exceptions. If you carry a knife clipped to your pocket, you are subject to arrest. NYPD arrests a lot of people for illegal visible carry. If you do meet one of the exemptions (ie. the knife is for use in the course of your employment, and you're on your way to work when you get arrested), be prepared to provide documentation from your employer when you go to court. You can be punished with a fine and/or up to 15 days in jail (IIRC).
Daggers and "Dangerous Knives" are banned statewide via the NYS penal code.
If an officer asks you why you are carrying a knife, never tell them it is for self defense. That will indicate to them that you intend to use it as a weapon, and even if the knife is otherwise legal, will get you arrested.
There is several previous threads that on the subject of NYC case law as it pertains to knives. It would probably be good for you to read up on them.
http://bladeforums.com/forums/showthread.php?t=538628
http://bladeforums.com/forums/showthread.php?t=563133
On a personal note- I live in Connecticut. When I travel into NYC, I prefer to carry a smaller (small stockman or peanut or a red SAK usually) slipjoint knife, deep in my pocket. I also use it as sparingly as possible. Virtually every locking knife can be flipped open- with the right technique and a little effort.