I will start:
asdf (to switch Elixir/Erlang versions easily)
tmux (to run tests and have multiple terminals)
Integrated terminal (vim, vscode and emacs has builtin)
Run tests from editor (vim-test/vim-test)
elixir-ls (https://github.com/elixir-lsp/elixir-ls#features)
Alternate between test and your implementation file (renderedtext/vim-elixir-alternative-files)
git client in editor (Magit in Emacs, Fugitive in vim and vscode has a builtin)
Open the current line(s) in GitHub from editor (renderedtext/vim-elixir-alternative-files)
ripgrep (to search in terminals and text editors)
Git blame (:Gblame with vim + Fugitive and GitLens in vscode)