Blog by Nati Shalom on scalability, databases, caching, and architecture, with Twitter as a recurring case study.
HN commenters include it in a list of recommended technical blogs, without comment on specific posts.
http://www.igvita.com/
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http://adam.heroku.com/
http://www.catonmat.net/
http://mechanical-sympathy.blogspot.com/
http://natishalom.typepad.com/