I don't use neck knives as often as I use to...mainly because I did a lot of traveling via helo in Iraq and I avoided anything around my neck. When around the house or bumming around the camp site, I do like a neck knife for quick access when needing a blade. My prefered method of carry is a Fallkniven WM-1 with a figure-eight 550-cord MercHarness. Now this is an easy way to carry/conceal a small fixed blade, yet is still quick to access. If under a shirt, you avoid the possiblity of it snagging as well.
Here's a few, some are no longer made (Left to Right):
TOPS Ferret
TOPS Sparrowhawk
Benchmade Snody
AG Russels Woodswalker (probably the best little neck knife for the money)
Benchmade's Tether knife (not sure if still made?)
REKAT Hobbit's Fang (no longer made)
Buck's Mayo Kaala neck knife (excellent knife for the price)
Fallkniven WM-1 (pricey, but a great small fixed blade)
BRKT Necker
Gene Ingram's Naket Hippo
Runningdog Pup (no longer made that I know of)
RJM Handiman
At the top (Left to right):
Buck Hartsook (small!)
Bark River's Slither
Some variation of the Bird and Trout finger knife
Emerson's LaGriffe
J.W Wharton Stag & Steel (more of a "pocket" knife as it has a thick handle)
ROCK6