dust, crap, scars and spider webs... but seriously:
free software T-shirts (mostly from conferences and GBs, but also one that was given to me by Marble developers), red hats (I have two) and baseball caps (one still probably has Linus Torvalds' dandruffs, lol)... and penguins (i.e. Tux)
backpacks (seriously, I have at least ten backpacks, although only Wenger Maxxum and the crumpler one are used regularly... it's almost like purses)
music (mostly European power metal, nothing extraordinary except some Helloween bootlegs)
pictures of weather, especially clouds or fog (unfortunately, I haven't had time to process it, it's like ten thousand photos)
headphones (proud owner of HD 25-13 and HD 565 (the 90s!), I want to get rid of the rest (cheap grados, audio-technicas, musician-centric beyers))
keyboards, mouses and trackballs (nothing too exciting compared to other members... and again, I want to keep just a few ergo boards and trackballs, and be done with it)
books (not physically, but I've read almost JRR Tolkien's writings including the less known ones)