Those which I check regularly that most often cut to the core of Biblical Greek concerns are:
Greek Language and Linguistics - Micheal Palmer (
http://grklinguist.wordpress.com/)
Included here is a good introductory online primer that's unfinished but very well-developed.
Hypotyposeis - Stephen Carlson (
http://hypotyposeis.org/weblog/)
Title ("annotations") taken from Clement of Alexandria: highly specialized, usually concise notes on recondite matters
NTDiscourse - Steve Runge (
http://www.ntdiscourse.org/)
Focus on the HOW of Biblical Greek discourse delivered with a gratifyingly serious effort to avoid linguistic jargon
NT Resources Blog - Rod Decker (
http://ntresources.com/blog/)
A variety of observations by one of the best seminary teachers of Biblical Greek, often bearing upon his work on the Baylor commentary on the Greek text of Mark's gospel
ΕΝ ΕΦΕΣΩ Studies in Greek Language & Linguistics… - Michael Aubrey (
http://evepheso.wordpress.com/)
Little or nothing about Ephesians (just as well, from my perspective) but very thoughtful observations and reports about reading on matters of Greek Linguistics
Biblical Language Center - Randall Buth (
http://www.biblicallanguagecenter.com/category/blog/)
Where Randall sets forth the hows and whys of a Greek pedagogy dependent on use of Biblical Koine as a vehicle of liv communication between living speakers and listeners, writers and readers
Better Bibles Blog - Wayne Leman et al. (
http://betterbibles.com/)
Frequently expounding why some versions convey more accurately and clearly the meaning of particular GNT (and OT Hebrew) texts. Iver Larsen has had some fine entries here.