(На русском: Пособия по математике)
Here you can find a collection of trigonometric formulas, you need to know by heart for solving any trigonometric exercises.
Formulas for calculating the sines are interesting too. These formulas can be used for calculating a lot of sines.
Also, there are some approximations of pi with different accuracy. Sometimes the application of these approximations can be very useful.
Online service "decomposition of the number to a multiplier" allows to decompose any given number to primes multiplier, as well as to all possible pairs of multiplier.
Online service for solution of equations allows to solve the equations of the second, third and fourth power.