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US Stock Market - volume based real-time alert 
Real-time equity (or any other market) data is not available for free anywhere in the US.  It is always delayed by 10-15 minutes. On the other hand, online brokers who target the "day trader" (Interactive Brokers, TD Ameritrade, etc.) offer much closer to real-time data AND feature all the tools/alerts/charts/etc. you could ever possibly dream of.  I bet the type of alert you're asking for is available with just a couple of clicks on one of these brokers' platforms. Of course, accounts with these online brokers are not free; you must pay for these sophisticated tools and fast market access.  Another down side is that the data feeds sent to you by even the most sophisticated online broker are still delayed by tens of seconds compared to the data feeds used by big banks and professional investors.  Not to mention that the investment arm of the broker you use will be making its own trades based on the data feeds before relaying them on to you. So this begs the question: why do you need real-time information?  Are you trying to "day trade" -- i.e. profit from minute-to-minute fluctuations in the stock market?  (I can't in good conscience recommend that, but best of luck to you.)  If on the other hand you don't truly need "real-time" data for your application, then I support @ChrisDegnen's approach -- use public data feeds and write your own software.  You probably will not find any free tools for the sort of alerting you're looking for because most folks who want these types of alerts also need faster feeds and are therefore already using an online broker's tools.