Date Calculator Tells the number of days between two dates, or defines a date based on the number of days from a specified day.http://cgi.cs.duke.edu/~des/datecalc/datecalc.cgi  site info 

AKiTi Math solving resources for quadratic, cubic and quartic equations, along with utilities for rootfinding, maxima/minima of functions, N equations in N unknowns, numerical integration, data fitting and eigenvalues and eigenvectors. 
Calculator Tab Scientific calculator with features including long term memory storage, ambiguous input filter, possibility of switching between comma and point as decimal separators, and saving of settings between sessions. 
PHPSimplex A free tool which enables users to solve linear programming problems. The tool has no limits on the number of decision variables and no restrictions on the amount of problems. 
 