Rope Cutters

A lot of BDSM advice, especially for rope and shibari recommends the use of EMS safety shears for emergency cutting of ropes.  These are designed for cutting quickly while keeping a flat, non-sharp part against the skin.  They are way better than regular scissors so don't confuse them with normal scissors (which are not very good at cutting rope...especially in a hurry.)

You can get them here for $12  http://www.amazon.com/dp/B002L9QML4

3rd Best Option

While these are good and pretty cheap, these are very far from the best.  The best would be either a Karambit emergency knife (http://www.karambit.com/safety_and_rescue_tool.htm) for about $50

2nd Best

Or a Benchmade Model 5 Emergency Tool http://www.benchmade.com/products/5  for $35.00

The Best - get it. 

The benchmade is my choice as the best for BDSM emergency cutting tool.  It's small, easy to use and cuts rope extremely effectively.  It also comes with a handy little case that can be attached to the strap  of a rope bag  -which is really perfect as there is no digging around the bottom of your bag for it -- and it is also clearly not a weapon so you can get it past the bouncer at BDSM events in clubs. 

This handy little tool will cut ropes far faster than EMS shears: especially if you are using thicker rope. 

One thing about this though...don't use it for anything but emergency cutting -- you want it as sharp as possible when seconds count.