Wolfram Research
Wolfram Math World: Rhombus
This math site explains properties of the rhombus, gives a visual aid to help with the formula given for it's area.
Analyze Math
Analyze Math: Rhombus Problems
The geometry tutorial explores the definition, area, and properties of a rhombus. The activity has several examples with detailed solutions.
Quia
Ixl Learning: Rhombuses
Brush up on your math skills relating to rhombuses then try some practice problems to test your understanding.