Give a third party my password so it can scan my email for financial data? No thanks.

I realise there are people who would love this convenience, and you'd make a killing on targeted ads, but this is a privacy nightmare. Good luck getting people to trust you. Furthermore, you really want the results of the filtering to be applied in the user's own mail client rather than having a separate UI..

Might be feasible as a client-side app. How about a Thunderbird/Outlook addon with a subscription service for known filters?

(What is the Google Chrome of desktop mail clients, anyway? Hardly any seem to use WebKit.)