Hard to narrow it down to a single item, but here are a few:
External hard drive with pictures, school assignments
Box with racing numbers, ribbons, medals from various cross country and road races
Accordion folder with assorted ticket stubs, brochures and memorabilia from traveling
Photo albums (yea I actually still have a few

)
Forest scene my grandmother painted (hangs in my den)
I have way too much other crap in my place - books, games, movies, clothes, stereo, computer, kitchenware. No matter how much I get rid of, there's always tons of it.
But I'm finding that I care less about my material stuff than I used to - it doesn't define me. Sure, if it breaks I'll be a bit disappointed, but I'll move on.