Stocks can be traded online on various platforms such as:

1. Online brokerage firms - Examples include Robinhood, E*TRADE, TD Ameritrade, Charles Schwab, Fidelity, and many others.

2. Stock trading apps - Apps such as Robinhood, Webull, and E*TRADE offer commission-free online trading.

3. Stock trading websites - Websites such as Yahoo Finance, Stockcharts.com, and TradingView provide tools for researching, analyzing and trading stocks.

4. Stock market simulators - Platforms such as Investopedia and Wall Street Survivor allow users to practice trading stocks without using real money.

5. Robo-advisors - Robo-advisors such as Betterment, Wealthfront, and M1 Finance provide automated investing services, including buying and selling stocks.