National Council of Teachers of Mathematics
Nctm: Illuminations: Volt Meter
As you put the different type of batteries on the volt meter pay attention to the voltage equations in the table. See how positive and negative numbers interact.
BBC
Bbc: Bitesize Revision: Negative Numbers
You can learn to recognize negative numbers and do simple computation with negative numbers. The explanations are short, simple and clear. There is even a test to check your introductory skills.
Interactive Mathematics
Interactive Mathematics: Integers: Negative Numbers
Explore the world of negative numbers as you look at their relationship to positive numbers, integers, absolute value and more. Test your knowledge using he exercises and games provided.
Purple Math
Purplemath: Introduction to Negative Numbers
This lesson from Purplemath.com reviews negative numbers and their operations. Number lines are used for demonstration.