University of Vienna (Austria)
Differentiation 1 Maths Online Gallery
This is a wonderful site that helps students understand and practice with the concept of derivative as the slope of the tangent of a function. The site contains several interactive exercises.
Jim Loy
Derivatives
This site explains the derivative as the tangent at a point on a curve. The concept of finding the tangent is discussed; links are provided for the concept of taking a limit.
Interactive Mathematics
Interactive Mathematics: Tangents and Normals
Finding the slopes and equations of the tangent and normal lines of curves is demonstrated using derivatives. Interactive examples are provided.