Sure, Yahoo Finance rocks. But plenty of other, smaller sites offer good data for the online investor. Yesterday's San Francisco Chronicle outlined seven of them.
Domash has a good range of sites, such as a site that tracks Chinese companies that buy a U.S. listed company instead of going through an IPO, and another that lists potentially undervalued stocks based on its intrinsic value and a "margin of safety." He also has his personal favorite for that investment must-have, price charts.
Here's the online version of the story, with links to each suggested site.