Brooklyn based Ideajax has released an ajax financial desktop demonstration site that's already useful and has lots of potential for the future. Called SaneBull.com, it currently uses Yahoo! Financial news to display automatically updating information on stocks in near real time (15 min delay via Yahoo!). Updates are displayed every 5 seconds.
When the company rolls out personalized features and premium accounts in the future they will use a source of information that allows them to charge for use of their interface. Future versions will also include charts and other, richer data. If you want to see the site operate after US markets close, there is a test site available as well.
It's a great example of an entire site based on ajax, targeting a niche of users and in the end providing a useful service and enjoyable user experience. Serious investors will of course continue to use other more heavy-duty paid services, but for casual investors and people with light research needs – SaneBull could prove a compelling option. It's already fun to use.