List of models that I think are unused with filename (without .mdl) and description:
archer: Some kind of Xen fish?
breather: Could've been in HL but I don't recall it being there...looks like another Xen thing-a-ma-bobber.
chumtoad: Probably something Valve worked on but put away...it was in Poke646 and maybe a usuable weapon in Point of View.
construction: According to the skin names, it seems to be an alternate Barney or new character.
crystal: I don't recall this being anywhere...
dead_osprey: I've never seen a beat-up Osprey flying in HL.
doctor: I believe this was the old Gordon Freeman, but attempts to open it lead to a crash, for some odd reason.
filecabinet: When did they need FILE CABINETS as MODELS?
floater: Another Xen sea/floating creature?
friendly: Not really friendly...probably designed for a monster. Interesting how if you look at the skin face.bmp, you can see "TED" on it, and how there's a vomit animation. Looks like some Doom3 monster to me...
gasbag: Apparently not human. The skin even says "xeno".
glassgibdirty: When did they even have a dirty glass texture?
hassault: Like the one in Sven-Coop, it's one of those grunts with a giant spin-up Chain Gun or whatever they're called.
That's only a few of them...I'll post more later.