TradeSignal.com
(standalone)
and
tradesignalonline.com
(Web application, free for EOD data as far as I remember, good to try out compatibility with EasyLanguage)
TradeSignal (web and standalone) uses a language called Equilla, which is fairly compatible with EasyLanguage. But there are differences. They however have a huge number of indicators and strategies available, probably more than what comes with TradeStation:
Indicators