Teachernet
National College for School Leadership (NCSL)
Centre for the Use of Research and Evidence in Education (CUREE)
Department for Children, Schools and Families (DCSF)
General Teaching Council for England (GTCE). Also here and here.
Ofsted
Collaborative Action Research Network (CARN)
TDA - Training and Development Agency for Schools
The Coaching and Mentoring Network
The Boston Plan for Excellence. Also here.