I thought I would gather together useful sites/tools for checking your work.
This will be kept updated with any new tools I come across that I think will be useful. I will also add relevant notes and examples.
Do explore the various pages;
- Order of Operations
- Standard Form
- Factors
- Fractions
- Equations: Linear & Quadratic
- Sequences
- Trigonometry
- Statistics – Diagrams
- Statistics 16+
- Polynomials
- Calculus
- Matrices
- Decision Mathematics
- Mechanics
I have checked all links are working and made some happy discoveries along the way where tools have been improved. There are many calculators here from basic to rather more advanced. I will keep these pages regularly updated.
On Decision Mathematics for example, check the excellent Linear Programming grapher from zweigmedia.com:
Linear Programming Grapher – zweigmedia.com
And look at the calculator from zweigmedia for normal distribution probabilities on the Statistics 16+ page, not only are probabilities calculated but a very clear diagram illustrating those probabilities is also provided.
Normal Distribution Calculator – Random Science Tools and Calculators
The Graphs section has also had a major update with all the Desmos resources gathered together
Mathematics for Students 
What a great resource! Thank you.