My opinions on neck knives may not match others, but are right for me personally. Some or all of this may make sense for you:
Neck Knives ...
1. Are backup knives, not primary defense or utility tools (99.5% of the time). I prefer pocket carry for primary (98% of the time).
2. ... must not print under a suitably loose fitting shirt.
3. ... must have 2-1/4" to 3-1/4" cutting length, which means 5" to 6-1/2" overall length, to stay reasonably small and light. See #2 above.
3b. ... must be light weight. This argues for thin blade stock, 1/8" or 5/32" is fine. 3/16" is getting thick.
4. ...must be corrosion resistant, and that says D2 at a MINIMUM in terms of chrome content/corrosion resistance (RJ's Handiman would be better out of ATS-34 or BG-42 or CPM 4x0V, darnit). Salty sweat will assuredly rust your blade, or the edge if black coated, if you carry much at all.
5. ... should offer a secure grip, must have a built in index finger "guard" of some kind, usually in the form of the knife (Ralph ArcLite) or cutout (RJ's Handiman, Crawford skeletonized).
6. ... must not offer an assailant an easy way to choke me. That says you use a breakaway "keychain" type chain, or use paracord attached in one spot with a compression fitting that will allow easy breakaway.
7. Must have a secure, lightweight kydex inverted carry sheath. Kit Carson's F4 neck knife sheath with adjustable Chicago screws with rubber grommets is a gem.
There are many other good ones, but the neck knives I own all fit the above criteria anywhere from pretty well to completely:
Darrel Ralph Arclite custom CPM420V
http://www.darrelralph.com/ddr/straight/cfaalslt.jpg.html
http://www.darrelralph.com/ddr/straight/arc5w.jpg.html
Kit Carson small F4 CPM420V
http://www.robertsoncustomcutlery.com/kitneck.htm
Pat Crawford skeletonized ATS-34
http://www.crawfordknives.com/html/NeckKnife.html
Larry Chew NeckBone
http://www.knifecorner.com/larrychew/pictures_3.htm
Brian Tighe ALO
http://www.tigheknives.com/Tpg5Tac.html
Emerson LaGriffe (very small, wicked and clever little slasher claw, not much of a stabber):
http://www.emersonknives.com/Tact_index.html
Mike Yurco (can't find on web)
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A couple threads that might be of some use:
http://www.bladeforums.com/ubb/Forum64/HTML/002940.html
This thread has pics:
http://www.bladeforums.com/ubb/Forum3/HTML/002592.html
You might search with the word "neck" and find more.
[This message has been edited by rdangerer (edited 03-30-2001).]