100Mac is a simple 100-cell calculation program.
100-cell calculation is invented in order to develop a fundamental mathematic capacity of elementary school students in Japan. There are 10 numbers in first row and 10 numbers in first column. Simply add two numbers from the top and the left (10~10=100 times), as shown in the screen shot.
It's devised for small kids but it's also good for adults who want to keep their brain younger and healthier. When you feel sleepy or tired, try to make a calculation. As a result, maybe your mind will become much sharper.
If you feel it's too easy, you can download a source code so that you can adjust the program to your own capabilities.