Yahoo on Monday unveiled the first meaningful work it has done to turn itself into a social platform: a social in-box for Yahoo Mail, third-party applications on its My Yahoo start page, and new a Yahoo Toolbar.

Yahoo's new open platform, referred to as Yahoo Open Strategy, or Y!OS, represents the company's attempt to reinvent itself as a social network.

The company introduced Y!OS in April with the launch of Search Monkey, its search developer platform. Other parts of Y!OS, like the Fire Eagle geo data platform and new Yahoo account profiles, have surfaced but have had relatively little impact on Yahoo's users.

Monday's announcement brings a major change to Yahoo Mail, the company's most widely used service.

Yahoo Mail now allows filtering based on one's Yahoo friends, which Yahoo now calls Connections. In time, it will also allow filtering based on one's Yahoo Mail contacts and other criteria.

Designating someone as a Connection is a process similar to "friending" someone on Facebook.

Yahoo Mail now includes a Welcome page with messages from Connections and with Update notifications from other socially enabled applications.

"We've long talked about mail being a dormant social graph; this is the first time we're exposing that in a very big way," said John Kremer, VP of Yahoo Mail. "This is a step toward prioritizing the in-box in all the ways that our customers want."

Yahoo is also changing My Yahoo and Yahoo Toolbar so they work as social applications.

My Yahoo is now a lot more like iGoogle: It's a Web page that users can personalize with embeddable applications from Yahoo or third-party developers. And just like iGoogle, My Yahoo users will have themes, through the new My Yahoo Themes API.

The difference between My Yahoo and iGoogle, said Cody Sims, senior director of product management for Y!OS, is that the Yahoo apps can reach across Yahoo's platform rather than being restricted to one area like iGoogle.

The new Yahoo Toolbar will be offered as preview software later this week. It provides easy access to Yahoo Mail, other Web-based e-mail providers, and other applications regardless of where one has roamed on the Net. It will include Updates on the activities of one's Connections and will provide new search suggestions and vertical search options.