Gradients
Y = mx + c
and Equations of Tangents and Normals
Watch this first - explaining how to draw a graph from it's equation
Plotting Straight line Graphs - Interactive Practice
Maths is Fun - especially if you scroll down and try some more of the options on this explanation page, you can move the lines yourself and learn how to calculate line equations from various different pieces of information
Or try the Waldo Applet - much easier to understand and use
A Real Grownup Worksheet - explains the concept of straight line graphs and their equations with a few questions too
Explaining Inverses - very complex stuff