“The universe is under no obligation to make sense to you." - Neil DeGrasse Tyson
Quick Graph - regression models - fitting the best-fit-line to data (also MyCurveFit, GraphPad, StatsBlue, Desmos, NCESGraph).
NIST Uncertainty Machine - the easiest way to use the awesome Monte Carlo method. Uncertainty Calculator - calculus method - A-level exams ask for 'correlated' i.e. pesimistic uncertainty estimates. Another semi-decent calculator using the calculus method.
Alcula - simple descriptive statistics, data dispersion/box plots (also StatsKingdom).
Distributome - probability calculator and simulator.
WolframAlpha - SI units and prefixes, conversions, constants, data etc. using AI - just type a question!
Desmos - easy yet powerful scientific grapher and equation solver (also, a useful Desmos calculator; quadratic equation solver; GeoGebra; Mathway; Eigenmath; Calculator.net)
Engineering Toolbox - source of well-researched data on properties of materials, calculators and more (also HyperFactBook).
Periodic Table - interactive periodic table from the Royal Society for Chemistry.
2D Vectors Calculator - add multiple vectors and resolve them into components.
Triangle Solver - solve any triangle for missing angles and sides.
Omni - over 145 problem-specific calculators, with explanations (also vCalc, CalcSoup, Psychrometric Charts 1, 2)
G-Docs, Sheets, Forms and Slides - Your free Google office, all in the cloud (needs a login).