I have a CRKT Bear Claw neck knife that I have been rather fond of. Its well designed, light, discreet. Within their limitations, they can be useful, especially when carrying other knives may be problematic. I always bring mine to the beach. I prefer having a blade handy in the ocean (getting tangled in old fishing line, needing to cut retention cords on boogie boards) and without the neck knife, my only other options are clipping my folder onto my bathing suit and spending the next week getting sand out of the lock or strapping a dive knife to my leg, which can be a bit much for a few hours at the beach.
I got a LEO friend of mine a Cold Steel Mini-Colloden neck knife that he now wears with him both on duty and off. They are not spectacular self defense tools, but in a jam, and skillfully employed, they make a decent back up for something bigger.
Most manufacturers make one and, because of their size, they are often more affordable on the higher end than many other blades from the same maker.