The stockmarket is primarily driven by the news, both on the ultra-short term (minute scale) and at the scale of a few days. So, scraping the web and performing semantic analysis of investor sentiment can be a good way to get an edge in the game.

Though in order to be really effective you'd need to do it before Wall Street traders' reactions adjust the prices, ie. get access to Bloomberg's B-Pipe [1], do real time semantic analysis and place orders with ultra low latency. Which quite a few trading firms are already doing...

Other factors include P/E ratio (only accurate in the longer term), div yield, recent growth...

[1] http://www.bloomberg.com/enterprise/enterprise_products/data...