Following up on the Yahoo Open Strategy announced in April, Yahoo this week opened up its annual "Hack Day" to outside developers, while also giving glimpses to journalists of the forthcoming new features on its home page and various other properties.

"[Yahoo Open Strategy] platforms will harness Yahoo!'s unique strengths -- our rich and relevant user experiences (we're #1 in 7 verticals), our massive audience (half a billion users/month), and our deep data repositories (content, content, content) -- and open them to the innovations of the developer community. Our aim: to fundamentally transform how people experience Yahoo," wrote Cody Simms of Yahoo Open Strategy's product management group, in a blog post introducing the strategy last spring.

Then, earlier this week at the CTIA industry wireless show, Yahoo announced intentions to internally develop iPhone applications, meanwhile pre-releasing two new tools for outside developers.

Also being used internally at Yahoo for the iPhone apps, the new tools are geared to the creation of applications can run across Windows Mobile, Java, and Symbian environments with only minor code tweaking.

In a a press event this week at Yahoo headquarters in Santa Clara, CA, officials got more specific, stating that Yahoo will work with outside developers to produce new e-mail applications.

The e-mail apps will be aimed, for example, at letting users send photo albums or online invitations -- or search non-Yahoo content such as help wanted ads --from directly inside their mailboxes, the journalists were told. Also at the press briefing, Yahoo outlined plans to redesign its home page and other Internet "starting points" so as to make it easier for end users to access third-party content. For instance, users might be able to download music from Amazon.com from within Yahoo Music, officials illustrated. Additional services will be provided via widgets that can be added to the Yahoo homepage.

Now, at today's "Open Hack Day" -- the first edition of Yahoo's annual "Hack Day" ever to open its doors to anyone other than Yahoo employees -- the company will start to collaborate with developers on those e-mail and widgets projects.

Yahoo isn't the only online service provider making a bid for greater openneness this week. AOL also announced plans to open up to third-party content and services, starting with giving users the ability -- as of this week -- to view Yahoo Mail, Gmail and Hotmail from AOL.com.