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Data source for historical intra-day bid/ask price data for stocks? 
Interactive Brokers provides historical intraday data including Bid, Ask, Last Trade and Volume for the majority of stocks. You can chart the data, download it to Excel or use it in your own application through their API.  EDIT: Compared to other solutions (like FreeStockCharts.com for instance), Interactive Brokers provides not only historic intraday LAST**** trades **but also historic BID and ASK data, which is very useful information if you want to design your own trading system.  I have enclosed a screenshot to the chart parameter window and a link to the API description.