Khan Academy
Khan Academy: Algebra: Complex Numbers
Video explains what complex numbers are and shows how to write them. Includes an example of writing a real number as a complex number. [2:21]
Khan Academy
Khan Academy: Algebra: Complex Numbers: Introduction to I and Imaginary Numbers
Video introduces i as the imaginary unit, gives two versions of the definition for i, and then shows what various powers of i equal. [5:20]