Northeast Parallel Architectures Center, Syracuse University
Npac Reu Program: Tangent Lines
The Northeast Parallel Architectures Center at Syracuse University offers a fairly detailed site explaining the slope of a tangent line to a curve.
Other
Karl's Calculus: Derivatives: The Equation of the Tangent
This site goes through a step by step example of how to find the equation of the tangent to a curve using the derivative.
BBC
Bbc: The Circle
The equation of a circle is the focus of this site. Topics include tangents, intersecting circles, and intersection of lines and circles. Examples and a practice test are available.