Yahoo has invited more than 500 outside programmers to build new applications with Yahoo's services, and has dubbed the move "Yahoo Hack Day."

In opening up its email service, Yahoo is allowing developers to customize the visual aspects of the service, as well as create mash-ups with Yahoo's other properties, such as Flickr and del.icio.us, Reuters reports. In addition, versions of the service could be created to allow users to read email from multiple accounts and providers in the same browser window. Yahoo expects to make the API for Yahoo Mail available by the end of the year.

The move to appeal to developers comes as other companies, such as eBay, Google and Amazon, are making the technology behind their programs and services freely available. The benefit is that developers could potentially create new versions or enhancements of existing products or programs, which the companies could make available to the public.