Quandl is free for end-of-data data and of very high quality, but intraday is paid. If you need free intraday data (1-minute or 1-hour intervals) you can try firstratedata

TickData.com has the highest quality tick-by-tick intraday data but is very expensive and needs to be purchased separately for each ticker symbol.

Quandl is free for end-of-data data and of very high quality, but intraday is for paid subscribers only. For one-off downloads, you can try   Firstratedata which has tick and 1-minute bars going back 20 years.

TickData.com has the highest quality tick-by-tick intraday data but is very expensive and needs to be purchased separately for each ticker symbol.