My complete Stargate DVD collection (SG-1, Atlantis and Universe).
My whole bookshelf is also quite nerdy, it includes everything in the "Dark Tower"-franchise (all books, the concordance and the comics), a couple of astronomy books and several about computer science.
Oh my goodnes, this is difficult, because many of my possessions are incredibly nerdy. But, some top picks:
•I have a pair of earrings shaped like DNA double helices. No joke.
•My Red Blood Cell plushie
•My "Weasley is our King" T-shirt
•A polymer clay figurine of the Eleventh Doctor that I made :) (and various other fictional characters, but the Doctor is my favourite, and possibly the nerdiest)
Probably Darth Vader's TIE Fighter in Lego form. Or the Burrow in Lego form. And my Time Turner.
I have Voldy's wand, and a Hufflepuff scarf.
I have a thirty-foot scarf that I knit out of a VHS copy of 2001: A Space Odyssey.
I'm gonna say my bright orange Dr. Suess How The Grinch Stole Christmas Hi-Top Converse
Since I can't choose between the three - my Spanish translation copy of The Princess Bride, a cuddly Companion Cube, and a wand which may not be a Harry Potter replica, but uses infra-red and can be 'taught' to do things like change the TV channel by repeating the same movements. Sadly, saying 'wingardium leviosa' had no effect, but it's still pretty ace. Nerd presents are the best.
I love my quite nerdy posters. I have a poster of the Andromeda Galaxy above my bed! :)
My entire room dedicated to Star Wars.
Tenth Doctor sonic screwdriver, TARDIS USB cable converter; "It's okay, I'm from the Internet" t-shirt and my Sherlock plushies.
Plus, many copies of the work of Sophocles and Sapphos around my room somewhere.
Large Dalek figurine (about 30cm). Pulp Fiction figurines (plus my DVD case is made to look like the suitcase and shiny gold inside). Marauders map. Sirius's, Hermione's and Luna's wands. Giant Sirius WANTED poster. Danger Days raygun. Various blind box anime figurines. Jhonen Vasquez related plush and figures - Invader Zim, filler bunny. And what I like to think of as quite an impressive DVD collection for someone my age on my income.