Check out the Khan Academy video on calculating percentages.
A great site for Mathematics is The Story of Mathematics. Check out their written explanation on percentages.
Baisc math tutorials: Addition, Subtraction, Multiplication and Division, Algebra, Decimals, Percents, and Fractions.
Everyday Math from OpenLearn. This is a British site so they refer to "maths".
Need a review on dosage calculations? Want to do some exercises to test yourself?
DoageHelp.com is a nice resource. There are a lot of ads on the site but if you stick to the left hand menu it is easy to naviigate