Have a whole walk-in closet full of stuff. Sometimes I give them to friends/family. Have built whole PC's with left-over parts and given to aunts, uncles, nieces, nephews, etc.
It's also nice to have some parts around that you know worked before in case an upgrade gives you or a friend problems. Used PC parts aren't worth much, and giving them away to help someone or using them to troubleshoot is worth more to me than the few bucks I'd get if I sold them.