The Cardsharp4 from Iain Sinclair is the latest generation of his folding metal credit card knife. Originally designed as a last ditch, lightweight surgical knife that's easily disposable, the Cardsharp4 is CNC-machined from an wafer-thin strip of aluminum that, despite weighing only 24 grams, maintains enough robustness, strength, and precision to cut through anything from a tennis ball to a tomato.
The knife is built with a 2-stage, child-proof Zytel safety lock to preclude accidental opening, but when you do find yourself in a Cardsharp kind of situation, it will unfold effortlessly in just 3 elegant turns. The surgical blade features an extra long cutting edge at 65mm (conventional utility knife blades measure more in the 25mm range) and its supreme lightness trumps both Stanley and Leatherman blades (at 150 grams and 85 grams respectively) for easy carrying and storage.
June 2015 Update: We have had several email complaints from readers who have paid for a Cardsharp4 and never received it, so we are removing our link to its purchase page and categorizing it as "Discontinued".