Today's Wall Street Journal has an interesting Blog Watch discussion on linguistic and grammar blogs. The three blogs it notes all make good reading:
http://languagelog.ldc.upenn.edu/nll/ - With multiple posts each day, this blog covers a broad swath of linguistic issues.
MotivatedGrammar.wordpress.com - Linguistics graduate student Gabriel Doyle rails against linguistic prescription, or the notion that arbitrary but established grammar standards should be closely followed.
Spandg.blogspot.com - GrammarBlog approaches language oddities and mishaps with a lighthearted, irreverent tone. (And is headed with the tag line "Smug is good.")