US Stock Market - volume based real-time alert
TdAmeritrade offers this service for free using 3rd party company markit. From markit's site, below is their guarantee.  http://www.markit.com/product/markit-on-demand Markit On Demand delivers an average of two million alerts per day   through various technology platforms and via multiple channels,   including email, instant messages, wireless, RSS and Facebook.   Investors can subscribe to their alerts of choice, and Markit On   Demand guarantees that they will receive an alert within five minutes   of the event trigger for all price and volume alerts