I an not a "knife nut" or a collector, despite the fact that I used to be a fencing, sabre, and knife fighting instructor.  Still, I appreciate them as tools and, in some cases, blends of art, artisan skill, and functionality.

My only "collectable" is the Randall Model 18 "Attack-Survival" knife I had made for myself in 1978. (http://www.randallknives.com/catalog.php?action=modeldetail&id=6) I got this because: a) I'd fallen in love with the idea of a good sheath knife from Robert Heinlein's novel Tunnel in the Sky, and b) I did some survival camping in my college years (i.e., go out in the woods overnight with the clothes on your back and a knife).  

The Solingen 'cat' trench knife which I used as my "every day carry" in my 20's was unusual because of its combination of decent size and strength lock back blade and (more importantly) the fact that this knife was only about as thick as a comb-- and hence easy to forget about in your pocket until you needed it.  Reportedly unchanged in design since the days when it was World War II German army issue. (http://www.atlantacutlery.com/webstore/eCat/swords_and_knives/folding/lockback/cat_knife.aspx)

More recently, I bought another "Cat" (the original had disappeared), but also decided to pick up something beefier.  I bought an SOG "mini X-42  autoclip" (http://www.sogknives.com/x42mnac.htm) tanto-style serrated blade knife made out of BG-42 aircraft turbine steel, which is extremely hard (Rockwell 60-62C) but therefore brittle. Mine had its blade coated with Titanium Nitride (TiNi, a blackening treatment which supposedly helps the knife cut better but which probably just appeals to the Rambo-wannabe's).  I haven't found the blade to be at all brittle, though-- my knife has been used to cut wire, sheet metal, and (knife abuse here) to cut a fairly substantial hole in the roof of my house when I was installing a dryer exhaust vent.  (Granted, I had drilled a series of holes through the roof first, but that doesn't diminish the fact that the knife had to cut through 3-4 layers of shingles and 1/2" of plywood, sawing a ~10" circle!).  It holds an edge well enough that I've only had to sharpen it twice in a year.  The autoclip feature is the ability to adjust the tension on this knife so that it will adhere securely whether your pocket is thin nylon or thick denim.  This knife is lightweight, cheap enough that I'm not afraid to use it hard (as seen by the roof vignette) but expensive enough (~$50) to hold up.  My current everyday carry (EDC), I ended up purchasing a non-TiNi straight bladed version as well (they are out of production, so this seemed a prudent thing to do).  I gave another non-TiNi straight bladed version of this knife to my son to carry on his Philmont High Adventure backpacking trip.  http://www.philmont.com/page.php?page=site/info/camping.php&i_origin=Philmont

Based on my good experiences with the BG-42 steel and the autoclip system, I subsequently bought the full-sized (3.6" blade, vs. 2.9" for the mini) version of the autoclip (http://www.sogknives.com/autoclip.htm.)  It feels like a much larger knife than the addition 0.7" of blade would imply.  Also made out of BG-42 tool steel, I was pleasantly surprised that this knife happens to fit extraordinarily well in my hand-- arguably the most comfortable knife I own.  Depending on the occasion, it typically displaces its little brother as my everyday carry (EDC), especially on the weekends (when I tend to do home improvement projects).  Its straight blade offer significantly more cutting edge than the serrated mini-Autoclip does, and as mentioned above it fits naturally in my hand.  

The ultimate SOG knife.  I tend to favor small folders, but there are times when "size matters".  The SOG "Pentagon Elite II" is advertised by the company as the "Monster trucker" of folding knives.  I got one, and I would agree that it is a "WBK" (wonkin' big knife!)-- the only folder I've seen that has a handle large enough for my fairly big hand to fit on fully when using a Styers-style grip.  It features a symmetrical blade made of AUS-8 steel (Rockwell 57-58) that is fully five inches long and has a "sharks tooth point" that appears much stronger than a traditional knife point.  The knife incorporates SOG's version of the axial lock (arc lock), which I like.  The blade is heavy enough that once the arc-lock is depressed, it can be flicked open or closed with wrist action-- pretty slick.  This is the closest thing to a folding sheath knife I've ever seen.  Not an Every Day Carry by any means, but there have been times in the deep woods when it can be found tucked into the leg pocket of my fatigues... I've taken to calling it my '10 mile knife' -- the one I want in my pocket when I'm in the middle of a forest ten miles from the nearest house.

I also have a Benchmade 710 forged from M2 high speed tool steel-- another of those extremely hard alloys.  This is probably my favorite "pure knife" because of its Axial lock and efficient recurve blade.  It is a bit too large and expensive (I wouldn't abuse it the same way as the mini-autoclip) to be my EDC, but makes a dandy "slicer" for soft materials. (http://www.benchmade.com/products/product_detail.aspx?model=710)

I'd heard a lot about Emersons and their ability to take almost Randall levels of abuse, especially the CQC-7 series.  The CQC-7B looked a bit small, so I was fortunate to be able to pick up a slightly used SOCFK version on Ebay.  This features the same tanto-style blade as the CQC-7 (anecdotally able to punch through car roofs) in a larger handle.  The blade doesn't hold an edge that well or long, though, and I'm not as impressed by it as I would have expected to be based on the Emerson reputation.  I banished it to my car as my utility knife whenever I'm going on long trips near the water.  (Heck, if I ever end up driving into a river, I may need that strong blade to get out of the car. <G>)   (http://www.emersonknives.com/EK_Exclsive_Designs.html)

Back in my college days teaching Olympic style sabre and fencing, I picked up an Indian-forged cavalry sabre for use in workouts.  Definitely not a collectable!   When living in Brussels, I also picked up a 1918-vintage French infantry officer's dress sword for its historical value.

Knives closed:

Benchmade 710

Emerson SOCFK

X-42 Autoclip

X-42 mini Autoclip

Randall Model 18

Knives open:

Benchmade 710

Emerson SOCFK

X-42 Autoclip

X-42 mini Autoclip

Randall Model 18

Pentagon Elite II in a Styers grip-- a comfortable handful!  This is my 'ten mile knife' -- the one I carry when I'm ten miles from nowhere spending the night in the woods