External sources can be added by first installing the plugin, then referencing it in your config. Each source will have it's own setup instructions on its project page. Below is a listing of known external sources.
example: an example of how build a custom source for Neo-tree as an external plugin.
zk: navigate folder-sorted queries from zk-nvim.
neo-tree-diagnostics: Shows workspace diagnostics in a tree view.
netman.nvim: Neovim (Lua powered) Network Resource Manager. This is more than just a neo-tree extension but it also includes a source for neo-tree which can enable you to browse remote files.
neo-tree-tests-source: Neotest results viewer within neo-tree. You can run and visualize all the tests across your project.