This site has flashcards for addition, subtraction, multiplication, division, and many more.
Students can create their own flashcards.
There are also other math games. This site offers practice where you can choose difficulty and you have lots of choices how to display and scoring. These learning games are divided into age categories and are the Internet's more advanced cousin to the old-fashioned flashcards. This site has a database of mathematical problems for practice and sharpening skills. It is for grades K-6.