New York Stock Exchange. The US-half of NYSE Euronext.
NYSE has been a publicly traded company since 2005; at the time of its initial public offering, it merged with Euronext and ArcaEX.
While average daily trading volume on the NYSE is typically between 3 million and 7 million shares, only about 40% of this actually trades in the iconic Wall Street building. The rest trades remotely in regional exchanges.
In a good year, about 250 billion in new capital is raised from equity issues on the NYSE.