Heres a unique offering from Shomer-Tec. This is the Nickel Knife and its reminiscent of the coin blades once used by the OSS (Office of Strategic Services). This nickel has be designed to hold a discreet hardened stainless steel razor blade. It can be used to cut through plastic zip ties, rope, duct tape, etc., and any other escape and evasion applications.
From general observation you cannot tell the hidden blade. For best practice- carry this along with some other coins in your back pocket. That way if you are detained you can access from behind your back.