MyBlogLog was founded in 2005 by Eric Marcoullier and Todd Sampson, adding Scott Rafer as a third co-founder in 2006. It helps people "Discover who reads your favorite sites (including your own)". Bloggers can display photos of recent readers in a widget; readers get a profile page at MyBlogLog.com, now part of Yahoo. It's both a social network and a set of analytics:

We wanted to provide truly actionable data — where were people coming from, what did they look at and what did they click to leave?

What does Yahoo get? Marshall Kirkpatrick says it best:

MyBlogLog is an incredible model of how to leverage human psychology in order to access peoples' data. The prospect of seeing the faces of people who visit your blog is so seductive that thousands if not millions of people have offered their faces and information to MyBlogLog in order to participate.
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By acquiring this ingenious service, Yahoo! has gained access to the browsing history of scores of users and the traffic history of scores of websites.

Last week, Yahoo announced an API for developers, currently an invitation-only beta:

retrieve lists of readers, tags, and members of MyBlogLog communities, search for members, get a member's profile data including sites authored, tags, and identities on other sites, and more.

What does that means? Back to Marshall:

a website can offer personalized content based on a user's tags in Del.icio.us, Flickr, YouTube, etc. all via one easy to use service, MyBlogLog

Kent Brewster of Yahoo has already posted an example.