Mine, one of these Matrix Sentinel Figures.
A clipboard made from a motherboard that I took part in designing. 
A little leather “sleeve” for my fountain pen.
I figure it’s nothing more than a coffee table version of a pocket protector.
Printouts containing data of Western Electric test recordings of radio broadcasts circa 1924.
This is the nerdiest thing I own… But I love it and it sits high above my office! 
The Philosophy of the Lord of the Rings, sitting on my bookshelf, probably qualifies.
I’d throw in the Ring-on-a-silver-chain, too, but I can’t find it. 
Five different editions of Dungeons and Dragons manuals? A bunch of miniatures that I’ve painted myself? a souvenir six-side die from Gencon, the nation’s biggest Nerd Convention? A plush Cthulhu doll?
I dunno.
Daniel
Seasons 1-6 of DS9 on DVD.
I have a clock that displays the time in binary, using an array of LEDs.
Definitely these.
If it wasn’t for the location listed, I would swear you were one of my housemates!
(And if the location is a fib, and you are my housemate, gimme my Cthulhu back!)
I have a button pin that says “It might look as if I’m doing nothing but at the cellular level I’m really quite busy.” pinned to a plush toy mouse in my lab.
Is that a “Once More With Feeling” songbook I see, off to the left?
As for me, check it out:
http://www.geocities.com/bigbadvoodoolou/toys.html
Yeah, I have one of those, too. Well, binary coded decimal.
I also have 3x3, 4x4, and 5x5 Rubic’s cubes, plus a computerized one I programmed myself, and assorted other similar puzzles, a heliometer, a Plasma Sphere, a Galileo thermometer, a glass Klein bottle, approximately four computers, a Lego Robitics Kit, copies of Green Eggs and Ham and Winnie the Pooh in Latin, a CRC Handbook of Chemistry and Physics, a slide rule (which I know how to use), a homemade Darth Vader costume, complete with lightsabre, a 13-function utility pen, a coin vortex, two editions of D&D rulebooks, the Encyclopedia of Science Fiction, and three different Tolkien reference books.
Decide for yourself which of those is nerdiest.
This is a tough call. I have my badges on a rope by my desk as a reminder of DragonCon and working it. Working a scifi convention is pretty high up there…
I also have my dual monitor computer system, which is kind of nerdy to have that hardcore of a system.
I also have boxes and boxes of comic books in my room right now, a lot of which still need to be sorted.
I know I’m nerdier than that, I’m trying to figure out what is geekier.
I have shelves of Magic cards, as well as cards for other CCGs. Two dozen+ CCG books. Six different types of Cyberpunk roleplaying systems.
That’s a bit better.
A life-size cardboad stand-up R2D2. He’s a happy little fellow.
It is, just to the right of the Sunnydale High Yearbook.
I feel a strange mixture of pride and shame when I mention that isn’t remotely my entire collection.
The Box is gone but I still have the original D&D Books.
http://rdushay.home.mindspring.com/Museum/Fantasy/Covers/ODDcover.html
My boxed was taped like that. I also have the First Printing of the DM’s Guide and Deities & Demigods. The printing that included Cthulu and Melbonian Mythos.
Jim
I could name any number of books (AKA tomes) and paraphernalia, but I think “dice with more or fewer than six sides” pretty much sums it up.
My nerdiest possession? Probably a set of earrings bearing the emblem of the Romulan Empire.
Hard to say, so I’ll leave it to the reader to decide which is nerdiest:
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my 20th Anniversary (“Masterpiece”) Optimus Prime
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my sizable collection of comic books and graphic novels
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my grandfather’s slide rule (nerdhood runs in the genes)