Creately.com: diagram creator
Creately.com is a great website for creating diagrams for your lab notebook, website, report, etc. The site is really straightforward and gives you a lot of different options. It also allows you to save your finished products as a jpeg, png, or pdf, which is extremely convenient. Below is one example of a diagram I created on creately.com. [The majority of the diagrams in my report as well as my journal were created using Creately.]
LaTex Equation Editor: write equations in LaTex
For this online LaTex equation editor, you don't even have to know the codes for LaTex; just choose the symbol you want from the drop down menus and the equation editor automatically formats and renders the equations for you.
Hyper Physics: a little bit about a lot of physics
In my experience Hyper Physics is one of the best websites for explaining basic physics/optics concepts. The website covers a lot of different topics and is useful for gaining a basic understanding on a topic. They also include a lot of diagrams and links to related topics, which can be really useful in understanding a concept.
Linux dictionary: dictionary of Linux commands
I found this website to be a good 'dictionary' of Linux commands. I found it particularly useful if I knew a command, but wasn't one hundred percent sure what the command did.
Academia.edu: find scientific papers
This website is great for finding papers from a particular researcher. Just type their name into the search bar and their profile will pop up complete with a list of all the papers they have published and links to said papers. Another nice feature of the website is that it shows you how many people have viewed the researcher's profile and papers, so you can gauge the researcher's credibility.
Better Explained: easy to understand explanations of math concepts
I stumbled across this website while trying to understand Fourier transforms. This website is a god-send for anyone trying to understand the concepts behind the equations. The website covers from imaginary numbers to Euler's formula to the Fourier transform. The math it covers does tend to be a little more basic, but it's important to have a strong base to build on. With lots of diagrams and everyday examples, this is a great resource.