"We really believe opening up Yahoo Mail will have a game-changing influence on the industry," Kremer said. Developers with Monday's rollout gain access to documentation and guidelines for building applications that run on the new mail canvas, according to Yahoo.

A limited beta testing program has started for bringing in third-party applications into Yahoo Mail. Users will be able to access these applications from within the Yahoo Mail interface. Initial applications include Family Journal, for building a family tree; Flickr, for photo-sharing; Flixster, for sharing movie show times; WordPress, for posting photos and links to a WordPress blog; Xoopit, for sharing photos stored in e-mail, and Yahoo Greetings, to send e-cards.

Also, Yahoo Updates is being integrated into Yahoo Mail.

The new Yahoo Toolbar, available in a sneak preview later this week, checks new e-mails from Yahoo and other Web-based e-mail providers, monitors and searches for items on eBay and finds local movie show times. The toolbar also alerts users about Updates from personal connections.  Personal search suggestions surface the sites a user has visited and provides search results in the toolbar.

With the My Yahoo update, users can enhance personal start pages by adding applications created by third-party developers via the Yahoo Application Platform. Applications will range from productivity tools to games. The company also is launching a My Yahoo Theme API for third-party designers to build and share their own look and feel for My Yahoo pages.