On Bitfloor, you actually get up to 0.1% rebate on limit orders. From the docs:
"Any order that is on the order book will receive a rebate when filled. Think of this as a negative fee (you actually get extra funds). This is done to provide an incentive for some traders to place their orders onto the order book for others to better understand market conditions."
Same thing happens on the equity markets, with the difference being pocketed by the exchanges (they'll pay .29 cents per share to liquidity providers and charge .30 to liquidity takers)
"Any order that is on the order book will receive a rebate when filled. Think of this as a negative fee (you actually get extra funds). This is done to provide an incentive for some traders to place their orders onto the order book for others to better understand market conditions."