At our workshops and during other customer interactions, we are usually asked for links to good blogs and reading materials. Here is a list of my favorite SQL-related blogs, arranged in no specific order.
http://blogs.msdn.com/craigfr: Craig Freedman on Query Processing (QP)
http://blogs.msdn.com/sqlprogrammability: Plan cache, parameterization etc.
http://blogs.msdn.com/sqlserverstorageengine: Storage Engine
http://blogs.msdn.com/repltalk: Replication support team
http://blogs.msdn.com/psssql: CSS SQL Support team
http://www.sqlskills.com/blogs/PAUL: Paul Randal with lots of tips
http://blogs.msdn.com/sqlqueryprocessing/default.aspx: Query Processing again
http://blogs.msdn.com/sqlserverfaq: Other CSS SQL Support team members
http://blogs.msdn.com/davidlean: Good series of posts on SQL Spatial features
http://blogs.msdn.com/sql_pfe_blog: Our global SQL PFE team blog
http://blogs.msdn.com/robertbruckner: Good information on SSRS
http://www.sqlcat.com: The SQL Server Customer Advisory Team (SQLCAT)
http://blogs.msdn.com/sqlreleaseservices: Learn about the latest releases (Service Pack / Cumulative Update) from SQL product team
I will be updating this list as and when I remember / discover other useful blogs. I hope this is useful for the community!