Khan Academy
Khan Academy: Graphically Add and Subtract Complex Numbers
Add or subtract complex numbers, and plot the result in the complex plane. Students receive immediate feedback and have the opportunity to try questions repeatedly, watch a video or receive hints.
CK-12 Foundation
Ck 12: Polar Form of a Complex Number
[Free Registration/Login may be required to access all resource tools.] Here you will learn how to convert complex numbers from rectangular form to polar form. You will also explore the graphs of complex numbers on a polar graph.