F.E.A.T.
(fast-access edit assistance toolset)
marked
online Markdown editor
rsted
online reStructuredText editor
gved
online Graphviz/DOT editor
drawio
online flowchart maker
kolombos
CLI escape sequences/control chars visualizer
holms
CLI Unicode code point analyzer
watson
[publishing] CLI Unicode block viewer
Regexp engine comparison chart
general capabilities and edge cases
xterm-256 color groups
...by HSV channel values