I am looking for a sophisticated tool that allows you to define a
trading rule based on a stock chart event (ie a price crossing the 200
MA line). Tradestation has a really good product for this, but it
seems to be able to be tested only on a single stock at a time. I
would like be able to test a rule on a large number of stocks over a
specified time range. Furthermore, the rules that would be defined
should be defined in a language (such as EasyLangue from Tradestation)
to allow for complex rules. I am looking for either a web based or
software based application to make this happen. Suggestions?

See Quant Developer (www.smartquant.com). This is more of a toolkit
though, best leveraged if you have some development skills (written in
C#).