there are a lot of sites you can use, but i hang out on
http://finance.google.com/finance a good amount because i have my portfolio set up there.
another intersting site is
http://www.fool.com/ although you have to pay for a lot of their info
i'm also on
www.scottrade.com which i've used for a few trades but not much for research... i do believe it is pretty comprehensive as far as research and education goes though.