Spyderco has several knife designs that remain steadfast year after year.  What keeps people buying them over decades is their basic function but also that they're upgraded or refreshed with new technology and materials when they become available.  A prolific design in Spyderco's line is the Calypso. 

Its arrival over a decade ago as a mid-sized folder has spawned numerous variations and converted legions of knife buyers to the benefits of flat-ground, thick leaf-shaped blades.  This is a workingman's or woman's folder founded on the premise that a comfortable, usable pocketknife is one that feels like a natural extension of your hand.