A few ideas quickly came to mind:
- a mini email client implemented as a Firefox extension or a desktop app
- a mini email client implemented as an app/widget/module/flake/doohickey for web-based portals and desktop environments like Netvibes, Pageflakes, eyeOS and YouOS
- a mini-email client that would be available to users of my Send Tab URLs extension so that users can send a list of browser tab URLs via Yahoo! Mail (and thus overcoming the annoying limitation on Windows of not being able to support every email client)
Considering that I am not a Yahoo! Mail Plus user, and that such a limitation shuts out an entire class of end users that are happy with their basic Yahoo! Mail account and are not interested in paying for unneeded extras, perhaps the best that I can accomplish with this for the broadest range of users is a little desktop (web-based or otherwise) utility that tells the users how many unread messages he has.
And I'm not the least bit motivated by the $10 commission for referring users to Yahoo! Mail Plus - I'm a programmer, not a salesman.
Update - apparently, someone has already written a Yahoo! Mail notifier extension.