Well, if you liken our hobby to others of similar enjoyment, then it would be those hobbies that feature "aging with grace", beauty, adjustability (tweaking), functionality, and oftentimes investment.
Those who collect postage stamps, matchbook covers, Coca-Cola bottles, etc. need not apply.
But many collect vintage motor-cycles, arms of many varieties: e.g cross-bows, alcohol dispensers: including the beer steins above and beer taps (I just missed out on a HALES tap to match my "Hales Heavy" placard...over my T8's), table-top radios (I have 30 odd from Crosbys to Lil' Pups), cast-iron farm animal lawn sprinklers from the early 1900's (these can fetch thousands!) pool tables, musical instruments of course and on and on.
I like to pick-up the odd item that will accompany my audio stuff...like 1/18 scale CART and F1 cars (Nuvolari, Clark, and Ma. Andretti cars in particular).
What other unique collectibles do Audiogoners love to keep?