Re:Simpsons already did it. (Score 2, Informative)

Yahoo! does this already.

Are you sure? Read the fine print at the bottom of the Yahoo finance page next time:

I am - Yahoo! Finance Launches Free Real-Time ECN Prices. Of course, both the Y! and Google finance teams have been trying to do this for a while - in our case (obvious disclaimer: I work for Y!, although not on Finance) we have to thank Bats Trading Inc for providing us with the data. It's not universal yet, but the blog post above indicates that we're working on it.

Google don't indicate where they're getting their data from, but they've been fighting to do it for a while as well, and as they're starting out with NASDAQ, you can assume they'll be trying to roll it out to further exchanges and markets as well.

All good for the market, as is the competition.