If you look into the description of Nasdaq ITCH data, you see two types of order entries in the book: Message of Type A and F. Under type F, you will see the identity of the market participant (MPID). In other words, you know the firm that supplies liquidity. Why liquidity providers may disclose their identity? What are their motivation to do so? Do they get a higher maker fee?
The two types of orders are called "Attributed" and "Non-Attributed". Venues will sometimes provide incentives to encourage order attribution. For example, Direct Edge has their "Edge Attribution Incentive Program" which you can read about on their price list.
I believe NASDAQ has offered incentives for attribution in the past, but I don't think they do with their current price list. Attribution is used, but I wouldn't say it is wide spread. For example, here is the list of MPIDs that show up in the NASDAQ ITCH feed on 2013-02-06 (A random date I snagged from our data store. Also, ignore
Finally, there might be some requirement to attribute your orders if you participate in NASDAQ's QMM or DLP program. You might check the detailed descriptions to see if there is. Ultimately, a firm is only going to attribute their order if (a) there is no downside to doing so; or (b) the upside, i.e. additional rebate or qualification, is significantly large to negate the downside.
ADDED: To expand on the above. If you look at the message statistics for the same day, 2013-02-06, you see the following for adding new orders:
So you're looking at about a $3.7\%$ attribution rate for orders on this particular day.
My blog discusses this at length: https://mechanicalmarkets.wordpress.com/2015/02/16/protecting-client-interests-anonymity-in-us-equities/ . To quickly summarize, as Louis Marascio said, DirectEdge pays brokers to disclose their MPID. Another possible incentive is the broker trying to maximize their order's chance of execution; if by disclosing their MPID the market is alerted to an order having particularly low-alpha, then that order will be more likely to execute.