WYSIWYG choices:

Math:
http://www1.chapman.edu/~jipsen/mathml/asciimathsyntax.html

Text:
http://www.codersgrid.com/2013/09/04/medium-editor-ui-zenpen-grande-js-and-pen-editor/

4 medium.js forks:
http://jakiestfu.github.io/Medium.js/docs/
https://github.com/tholman/zenpen
https://github.com/mduvall/grande.js
https://github.com/sofish/pen
https://github.com/daviferreira/medium-editor (my favorite so far)

Very interesting implementations:
http://madebymany.github.io/sir-trevor-js/
https://news.ycombinator.com/item?id=6570042

StackOverflow:
http://meta.stackoverflow.com/tags/editor/info
https://code.google.com/p/pagedown/

The classic:
http://www.tinymce.com/
http://www.wymeditor.org/

The minority:
http://imperavi.com/redactor/
http://www.sceditor.com/