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 specific securities 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."