For me it is Starbucks mugs.Particularly the 50 state mugs.
It started out a few years back when I had left over money on my college food account I had to spend or it would go to waste and I bought the Ohio state mug from Starbucks. Well then I liked it so much I bought the Florida one, and then the DC one, and then the Kentucky one, next thing you know I had New York. The list goes on. Anyway I am determined to buy all 50 mugs from every state. Even if they do cost $12.95 a pop!