Just in case some people might not know where to find him: http://slatestarcodex.com/ (Remember to give parent comment your upvotes, not me, if you want to vote for him.)
Currently Scott Alexander has 41 votes and everyone else has a total of 28. That's pretty impressive.
FWIW, I unsubbed from MR shortly after subscribing. It kind of felt like reading through someone else's RSS feed, with small amounts of commentary which often assumed more economic literacy than I actually have.
Also, Kevin Simler's Melting Asphalt is great and has lots of insightful essays about things. Warning: Still doesn't have archives, you're going to need to go through the meta posts to read old things.
I like Eneasz Brodski's Death is Bad. Not as moralizing as the title sounds, has lots of fun book reviews.
I wouldn't quite call him a rationalist blogger, but I love reading Tim Urban of Wait But Why.
Andrew Gelman wrote Less Wrong's favorite textbook on advanced Bayesian statistics. His blog is here. Some good stuff like a recent post on why the hot hand actually exists which I still don't completely understand.
I wonder if anyone has made any effort to systematically catalogue academics who blog?
I want to check out more rationalist bloggers.
Please list no more than one per answer. That way I can see from the upvotes/downvotes which ones are the most popular (and hopefully which ones are most worth checking out).
I know that popularity isn't a perfect signal, but hopefully it is a better signal here than on the rest of the Internet.