Since visiting home last summer (UK), I've been obsessed with collecting re-useable shopping bags - Waterstone's, Carrefour, M&S etc. As long as they have interesting designs on them, I'm in.
Antique copies (from the 1800s) of my favorite classic books. My Charlotte Brontë collection is shaping up!
Also, I usually buy postcards whenever I travel somewhere new, so I have a few of those!