Mental Calculation

For the matematically inclined, this article from Wikipedia on Mental Calculations has some good info on how to do calculation in your head. The methods I liked the most are Indirect calculation, Using square numbers, 9 and 3 rules, Approximating square roots and Squaring numbers near X.

This article on calculating the day of the week of a given date is really good. It taught me how to calculate the day of the week of a given date.