Wolfram Research
Wolfram Math World: Dihedral Angle
This MathWorld page discusses the dihedral angle, or the angle between two planes that can be found using the dot product of the two normal vectors of each plane.
Ministry of Education and Universities of the Region of Murcia (Spain)
Ministerio De Educacion Y Ciencia: Elementos Basicos De Geometria Del Espacio
Interactive resources and activities dealing with perpendicular planes, perpendicular lines and the angle between planes.