Through active searching or by accident, I have stumbled upon a lot of pages that might be helpful to future STEM students. I have curated some of such pages below based on their contents.

Some Tips: 
- Educational sites (.edu) are good for solid information.
Web is not for concrete learning. Try reading books or papers instead.
- Try Google Advanced Search option if you know what you are looking for.

Mathematics:
For the beginner, the best resource for mathematics (and many other fields) is MIT open courseware (MIT OCW) courses. I encourage everyone to go through the following courses-


Other notable courses include Differential CalculusDifferential Equations, and Probability and Statistics. Gilbert Strang and Cleve Muller have offered a short course on Differential Equation, but in my view, it's really inadequate. For comprehensive treatment on ODE and PDE, I would recommend Advanced Engineering Mathematics - D. Zill

Youtube content creators often post videos on math related subjects, but these videos can be used as supplementary at best and unwatchable or misleading in most cases. I would suggest looking for formal courses first before considering such videos. Nevertheless, some sources do have helpful contents.

Khan Academy has multivariable calculus course that can be watched in parallel with MIT course for better visualization. 3Blue1Brown is one of my favorite content creators. Check out the videos!

One final note! Some universities have math resources that can be another great resource. But often they are hard to find and seems rather unorganized. But if you are googling for a topic, there are chances that such link might show up. Keep your eyes open. Also, if you feel comfortable with lecture notes rather than books, there is a pretty good chance that University of Notre Dame has something for you!

Microtechnologies & Bio-microtechnologies:
While I was eagerly waiting for nanotechnology to surface, microtechnology made tremendous progress. Due to my interest, most links given here are somehow connected with bioengineering and bio-microtechnology. 

For a gentle introduction to BioMEMS: https://en.wikipedia.org/wiki/Bio-MEMS. Wikipedia, as always, a good place to start. If you are looking for a more coherent approach instead of just summary, I would suggest:
https://compliantmechanisms.byu.edu/content/introduction-microelectromechanical-systems-mems

From engineering viewpoints, https://users.ece.cmu.edu/~koopman/des_s99/mems/ is a very good resource. Also, please check the links at the bottom.

A not-so-recent but very important advance in microtechnology, in my opinion, is micro-electrocorticography. Check this paper if you are interested: https://www.ncbi.nlm.nih.gov/pubmed/18429704

Some lecture notes on MEMS and related topics:

Material Science & Engineering:
For material science and metallurgy, the best resource that has it all would be this one-

For molecular simulation enthusiasts- 

And here is a short overview of computational nanotechnology-


Academic Research Advices