I do not yet have much stuff here, but be sure to check out these cool places:

Tcl/Tk

• Tcl/Tk and its mirror here in GitHub
• A lot of packages for Tcl can be found on the page of the "Github Tcl organization"
• fossil SCM and its mirror here on GitHub

Markdown

• CommonMark is the place to start when you want to work with any markdown project
• SubEthaEdit is a nice and light-weight macOS editor also supporting markdown including a preview feature that is highly customizable. It uses the showdown parser written in Javascript

My gists on Github

• see here