My residence permit!!! Not a card, but a sticker in my passport.
My National Library of China Readers Card. I'm now allowed into the stacks.
My Beijing Transit card. Good for subway (2 Yuan = 33 cents), bus (1 Yuan = 16 cents) or taxis.
And of course, my Wu Mart Shoppers Club Card. This was, of all of them, the most difficult to obtain! The woman who took my application couldn't understand how I said my address, I don't know how to write it in Chinese, so we just put down HER address. I hope the police don't find out!