YMail
I am, as of today, an official developer on the YMail.app project. Hopefully in the next couple of weeks you’ll see the app moving forward, with at the very least, a version that works with the latest version of Yahoo! Mail.
To make things more interesting, this is not an official Yahoo! product 
Nice once
Good luck with it. Am a GMail fan myself though :p
Exciting news, I’ll keep tabs and will be happy to volunteer my testing services.