Yahoo! is set to allow outsiders to create new services using the Yahoo! Mail program, in the broadest move the Web has yet seen to enlist independent programmers to build a company's products for it.

Yahoo! officials said on Friday they planned to give away the underlying code to Yahoo! Mail, one of the crown jewels of its business, in a bid to encourage software developers to build new applications based on e-mail. The move to open up the underlying code of Yahoo! Mail — used by 257 million people — is designed to spark development of thousands of new e-mail applications built not only by Yahoo! engineers but by outside companies and individuals. Chad Dickerson, head of the Sunnyvale company's software developer relations program, said he believed that the open approach to programming represented the biggest single Web software ever to be opened up for public development. "Yahoo! is a very large company but we can't build every applications that a user might want," Dickerson said in an interview at Yahoo! headquarters. "You can imagine tens of thousands of niche applications (springing) from Yahoo! Mail."

Software developers have traditionally kept careful control of the underlying programming code of their products and allowed outsiders to make only incremental improvements. In recent years, Web developers have opened up that process to encourage outsiders far deeper access to the underlying code. Open applications like Google Maps and Yahoo!'s Flickr have inspired a new wave of programming in which developers can combine software features from different companies to create what are known as 'mashups' — hybrid Web products. The company made the announcement ahead of a 24-hour 'Yahoo Hack Day', where it has invited more than 500 most youthful outside programmers to build new applications using Yahoo! services.

Hack is used in its original sense of 'creative programming' not illicit sense of breaking into computers. 'Hack Day' mixes Web programming competitions, overnight slumber party and a music festival where pop music superstar Beck has been hired to play a concert on the Yahoo! campus. Technically speaking, Yahoo! is giving away 'browser-based authentication' for its e-mail service for developers to build new applications. Currently only Yahoo! Mail and certain broadband partners like AT&T and BT are granted such access to the code. This will allow people to make custom versions of the basic interface, or look, of e-mail. Other uses may include tapping the information inside a user's e-mail program to create new ways of displaying the information to individual users.Since Yahoo! keeps absolute control of usernames and passwords there are no security risks, Dickerson said.

The event drew Dan Lindquist, 23, an unemployed recent computer science graduate from Olin College in Needham, Massachusetts. As an example of what Yahoo! is allowing programmers to do, Lindquist quickly conceived of the idea of building a more intensely visual way of reading e-mail. "This is totally new," Lindquist said. "It's interesting to me not because I can build something to make people more efficient, but because I can offer something whimsical." He hopes to allow e-mail users to use the photo stream of Yahoo!'s photo-sharing program Flickr to see visual clues of what's inside each e-mail. Mention of "cats" or "New York" would trigger relevant photos from Flickr. If successful, he will post his work on his Web site at http://danlindquist.net later this weekend.

Yahoo! is not alone in its effort to open up who can program Web services using its tools. Major Internet companies including Amazon.com Inc., eBay Inc. and Google Inc. to established software providers such as IBM and Microsoft Corp. have embraced such moves.Yahoo! Mail's code will be generally available later in 2006 said Jason Rupp, product manager for Yahoo!'s e-mail services. Rupp said he hopes other e-mail providers will follow Yahoo!'s lead and open up the code of their own programs.This could allow a 'mash-up' to be created that permits users to simultaneously read Yahoo! Mail, Google's Gmail and Microsoft's Hotmail from the same browser window rather than forcing users to sign into each e-mail system separately."There is just all kinds of things people could do," he said.