How to post to a remote WP blog?

Hi everyone.
I see that Digg.com and some other websites have a feaure, where they ask you for your wordpress blog’s username,password, and url etc, and then you can post to your blog directly from their site (without having to login to your wordpress admin area).

I was wondering how that feature is implemented. I’m a php programmer myself and very tech-savvy, so if you can give me any details on whether there is a function in the wordpress API which makes this possible, or anything else along those lines, that would be really great.

Regards.