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20 years' worth of Magic: The Gathering cards
Seriously, I've been playing since alpha. The last time I counted my cards (which took 4 hours) I had almost 300,000. And that was 5 years ago. It's getting to the point that I have to start getting rid of other stuff to make room for all of them. I have a 15 feet wide, 2 feet deep closet with just boxes and boxes of them. And they're all organized by Set, Color, and Alphabetically. I would literally kill myself if anything happened to them!
My movie and game collections have been ongoing for most of my life. I have Atari all the way up to PS3/360 games. Most of my older stuff is in a 'retro gaming' cabinet next to my TV to show off :-P And my movie collection is pushing 400 DVDs and Blu-rays.