Sourcetrail - "Free and open-source cross-platform source explorer" supporting C, C++, Java, and Python.
Diez - "Diez is a free & open-source developer toolkit for expressing visual styles that can be shared across codebases, native platforms, and teams."
Ruby Next - "Ruby Next is a tool for supporting modern/edge CRuby features (APIs and syntax) in older versions and alternative implementations. For example, you can use pattern matching and Kernel#then in Ruby 2.5 or mruby."
Joplin - "Joplin is a free, open source note taking and to-do application, which can handle a large number of notes organised into notebooks." Uses Markdown for formatting and can import from Evernote exports.