Some of the ones I follow are:
David BIE: David Ross is one of the leading thinkers on Project-based Learning. He is part of an organization, Buck Institute, comprised of, not surprising, great people who have an incredible impact on PBL. David's Tweets revolve around sharing quality information in the arena of PBL.
New Tech Network: NTN is a group of schools, nation-wide, who's culture includes following a standards-based Project-Based Learning curriculum. It's one of the quality approaches to school-wide pbl.
New ones that I'm exploring...