Yahoo! Mail now has a publicly accessible API! Check it out at: http://developer.yahoo.net/blog/archives/2007/03/mail.html

Not only that, Yahoo! is actually offering a monetary incentive for developers who manage to sell their premium accounts.

The following is taken from the official announcement on Yahoo Developer Network:

Yahoo! Mail Web Service developers can earn referral commissions by building compelling applications targeted at our premium email users. For a trial period, Yahoo! Mail is offering an incentive for developers to build applications using the full functionality available for premium Yahoo! Mail accounts. Specifically, Yahoo! Mail is currently providing a commission of $10.00 for every new Yahoo! Mail Plus account referred by approved developers