I'm surprised to see that no one has yet mentioned quote.com. They appear to me to have the most extensive intra-day and daily historical data. For example, I recently download intraday data for the SP (SP500 futures contract) from all the way back from Dec. 1997. I think if you check other vendors, the most they provide is about 6 months of intra-day data. Perhaps an exception to this would be Tradestation who have been promising to provide many years of intra-day data but, as I understand it, have yet to fulfill this promise. (Please correct me if I'm wrong).
You can get a subscription to quote.com (qcharts) for about $80/month for which you'll be able to download to your heart's content. If you need to automate downloading of data then you could use qcollector (do a google search to find the vendor, if you're interested.)
Richard