In the development of maniFEM (and of its manual) the following tools have been precious.
The GNU/linux operating system, of course.
The emacs text editor.
Nice features : one can open the same buffer in different frames with
C-x 5 b or C-x 5 f, this is useful for moving text around within one large file.
Search and replace limited to a region.
One can launch emacs in daemon mode then invoke emacs-client which opens very quickly.
Highly customizable : you can add your own shortcuts, useful especially when using awkward physical
keyboards.
Cygwin for turning a windows-running computer into a decent one.
UserLAnd and termius for working on an android-based tablet.
LaTeX with scrreprt (Koma-Script report class).
This script is useful for keeping track of numbering (of sections, paragraphs, figures) in the source of the manual. The script was crucial when the manual was written in plain TeX; now that I switched to LaTeX it is no longer essential but it is still useful (it allows to use labels like "sect 6.fig 2" directly in the LaTeX source file).
evince (on windows) for viewing PostScript files, Foxit PDF Reader.
gmsh, mainly for visualising meshes.
Eigen, a library for solving linear algebra problems.