Numerous powerful text editor exist such as Emacs or VIM. These two are powerful with yet a sharp learning curve.
More modern editor and say, more user-friendly, can be used for not-so-geeky-people (as me the author of this page). Among them, let us cite Notepad++ (windows), Atom (from Github (Microsoft) - probably be deprecated in a near future), Sublime (not open source).
If you do not know what to choose, then I (the author) can only recommend VS Code, developed by Microsoft but open source. Numerous extensions exist (see for example Pierre Marchand’s blog post) and are of high interest for the mathematician community:
- Latex Workshop: $\LaTeX$ support (coloring, indentation, cross references, …)
- Git is natively supported in vs-code but the following extensions improve the user experience:
- Python, C/C++, …