This is stuff which is not categorised under other headings. Mostly it is client-side.
Stand-alone page that lets you enter a sequence diagram in Mermaid syntax. There is a live diagram render, along with export as an svg or a simple (long) link.
DotNet Stack Trace Prettifier
Another stand-alone page. This one lets you paste in a .Net stack trace and it will do some basic prettying up.