In addition to machine learning, one of my main research interests is theoretical computer science. Following the success of MathOverflow and other Q&A sites, cstheory.stackexchange.com ("cstheory" for short) was born about a year ago, and I've been an active participant since its inception. In the last year, we've worked hard to make cstheory a valuable resource and to promote it. I'd say the efforts paid off -- the site is doing quite well, with over 5000 members.
Moreover, cstheory recently added a blog (Aaron Sterling and Joe Fitzsimons are editors), whose posts are on topics related to theoretical computer science and to the site itself. In one of the first posts, Suresh Venkatasubramanian, a site moderator and one of the people most responsible for its success, wrote a nice summary of cstheory's first year.
I've also volunteered to occasionally blog about the learning theory side of things. My first entry, on learning regular languages, was just posted. It touches on many interesting issues learning theorists regularly encounter. Enjoy!