Sources of historical data for back-testing day-trader systems?

Hi guys,

Can anyone suggest some good sources of historical data for back-testing day-trader systems? Ideally, I'd like to be able to select a stock and have data going back a few years (the more years the better, really - I'd love to test against some "Black Friday" type events too).

What sort of prices do these sources usually charge?

Are there any free sources I can use to cut my teeth on before forking out for the more extensive data sets?

Thanks.