How to Create Straight Lines
This tutorial shows you how you can do straight lines with the gimp, using a feature called the Shift Key. Straight lines are a convenient way to make things that aren't so terribly affected by the imprecision of a mouse or tablet, and to take advantage of the power of a computer to make things look neat and orderly. This tutorial doesn't use Straight Lines for complex tasks; its intended to show how you can use it to create quick and easy line effects.
Basically: click, hold shift, click again.