1. Online and Interative Maths/Stats/Programming
   • Interactive equation solving     • Interactive Linear Algebra     • Math and Stats Calculators     • Python Learning, Pytorch in 60 mins
   • Wolfram Alpha for Advanced Students     • Octave Guide 2020 (free and Matlab like)     • Short Octive
2. Great Saylor.Org (Maths) , Superb List of Maths eBooks as well as Various lectures.
3. Deep Learning: Online DL Book (free and great),   MIT Courses, Pytorch in 60 Mins and Machine Learning. And of course watch YouTube e.g. Numerical PDEs(MIT) or Nonlinear Programming
4. Applied Mathematics Books (Excellent) by Prof John H. Heinbockel (Old Dominion University, USA)
5. Introduction to Real Analysis (Excellent) by Prof W F Trench (Trinity University, USA)
6. Mathematical Methods for Economic Theory (Excellent) by Prof M J Osborne
7. Finite Element Methods - lecture notes (University of Colorado at Boulder, USA). FEM theory (on Youtube)
8. Differential Geometry:     ETH lecture notes,   Principal Curvatures,   Mean Curvatures or   Curvatures in One Doc
9. Linear Methods of Applied Mathematics: Orthogonal series, boundary-value problems, and integral operators by Evans M. Harrell II and James V. Herod.
10. Engineering Math:
    
    • Chain rule, Partial differentiation + Chain Rule 1 (see also Chain Rule 2) [Multivariable Calculus Online] + Implicit Functions/Differentiation
    • Notes on Partial Derivatives, Differentials, Chain Rule, Implicit Functions, Maxima/Minima
    • Fourier Series + Fourier Series / Examples
    • Laplace Transforms / Examples
11. Fast Fourier Transforms: Lectures on Image processing, and Digital Signal Processing (book)
12. Introduction to Applied and Pure Mathematics
13. Numerical Recipes (the full book plus programs in C and Fortran)
14. Introduction to Numerical Methods
15. Numerical Methods for Partial Differential Equations (online)
16. Linear Optimisation and Numerical Analysis ,     Nonlinear Optimisation:         Lagrangian Duality and Convex Optimization,         Backpropagation in Neural Networks
17. History of Mathematics and Lecture Notes
18. Computing: HPC Computing,   Intro to Maple,   Learn LateX in 30 mins,   Colourful LaTeX,   LaTeX guide + LaTeX color/graphs + A Beginner's Guide to Matlab
19. Partial Differential Equations
20. Applied Maths - PDE's and Integral Equations
21. Functional Analysis
22. How to present a short talk. See also Thurston's wisdom + Career in Maths
23. Computer Science Tutorials and IT Skills
    Programming: Microsoft Excel Tutorial, Advanced excel, WallStreetMojo Tutorial, Deep Learning in MATLAB, IT Programming, Fortran Tutorials, MPI Fortran guide and Excellent MPI red book + Learn Unix in 10 mins

    Python For Beginners

    Julia for Beginners

    C

    C++

    Fortran 90

    Parallel programming

    Computer Programs (Book)

24. Books to preview and buy: A First Course in NS of DEs by A Iserles (2009), CUP                         Online Unit Converter + Time Zones
25. Nice URL clean (for google searches)

Special thanks to colleagues who suggest new pointers or fix old links:
including Andrew Stacy