This is a selected list of sites that offer free pricing data. Many of these have deep historical data but this can be inconsistent. In addition, most free sites discontinue coverage when a company's capital structure changes. For example, Yahoo! Finance has data for United Airlines, but only for the period after they emerged from bankruptcy in 2006
The Library has a few print sources with stock prices, but coverage can vary.
The Harold Washington Library Center downtown has extensive runs of the Daily Stock Price Record. It is probably a better place to go for people who are looking up a few historical prices that are outside of our database coverage.