What sources of financial and economic data are available online? Which ones are free or cheap? What has your experience been like with these data sources?
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.