University of Saskatchewan (Canada)
Emr: Exercises in Math Readiness
Practice your mental math skills with this site, where practice problems in addition, subtraction, multiplication, and division abound. Introductory, intermediate, and advanced levels are available. Just type in your answer and you'll...
Lawrence Hall of Science
The Math Page: Skill in Arithmetic: The Meaning of Multiplication: Mental Calculation
If you are looking for a website that explains and defines the basics of mental multiplication with helpful hints then you have arrived at an excellent resource. There are even practice problems at the very end of the lesson.
Other
Curious math.com: Math Is an Attitude
This site offers numbers tricks to help multiply and divide large numbers mentally. There are also tricks for powers and roots of numbers.