https://tabatkins.github.io/symbols
Easy access to commonly-needed symbols that aren't on your keyboard.
It's a one-pager, and the list of symbols is in a JSON blob right at the top of the document, so it's trivial to clone this project and change the lists to whatever characters you prefer. You may need to go into your repository Settings afterwards and tell it to serve GitHub Pages from your master branch; I don't know if that sort of thing gets carried along on a clone operation.
As such, I will not be taking any suggestions on the displayed characters; the set currently displayed is purely what I personally find useful, and can and will change over time.