The stock market, particularly the NYSE and Nasdaq, is traditionally open between 9:30 AM and 4 PM Eastern. Over time, with the adoption of new technology and increased demand for trading, these hours have been extended to include what is known as pre-market and after-hours trading.
The first place investors should look to find information about pre-market and after-market activity is their brokerage account's data service, if they have one. Often brokerage information services provide the most detailed off-hour market trading data, and they usually come free with a brokerage account. Investors will often be able to not only trade within this time period, but also see the current bid and ask prices for a specific security and the change in price compared to a previous period's close.
If you don't have a brokerage account or your brokerage account doesn't provide this service, there are several free sites that give users access to pre- and after-hours market data. The Nasdaq website offers comprehensive quotes on shares listed on the Nasdaq, showing every trade – including the price, time and size of trades made in off-hours trading. For pre-market trading information, use the pre-market quotes service, and for after-hours information, use the after-hours quotes service. Although the NYSE's website does not offer such a detailed service, in terms of depth of information, the quoting service on its site shows you the last movements of the stocks during the off-hours market.
Other services like Yahoo Finance will show the last trade made in the pre- and after-hours markets. These services will usually cover all stocks – whether they trade on the NYSE, Nasdaq, or another exchange.
To learn more, see "Activities You Can Take Advantage of in the Pre-Market and After-Hours Trading Sessions."