IEX is a relatively new stock exchange that accounts for about 2% of daily stock market
trading in the U.S. You can see their exact market share on our page about U.S. stock exchanges.
IEX makes their open "book" available for free. The term book refers to all open (unexecuted) orders that are sitting on a stock exchange, waiting to be executed.
Normally, there aren't alot of open orders, because most orders are executed within a second, assuming that an exchange can match up a buy order (referred to as a "bid") with a sale order (referred
to as an "ask"). Most open orders sitting in their book are sitting there because they are "unrealistic" --- someone who is asking to sell for too high a price, or someone hoping to buy at too low a price.
The CBOE exchanges, which have about a 16% market share, also have a free CBOE book viewer.