Making open source technical analysis software

Traders and investor who rely on technical analysis need tools and software to assist them with decision making. Such commercial tools do exist, but what if i want to build my own?
I want flexibility in my software, I want to be able to do backtesting on various indicators, I want to discover reliability of particular indicators in Australian market.
My assumption is if technical indicators are measuring market’s state of psychology, then surely, different market have different behaviour in their way of trading.
I think there is a potential for further study here, to learn about the Australian market. I don’t think there’s such data mining software for technical analysis yet. If i can code such a software myself that would be great, but I need free historical data. Where can i find them? 😛