Nix for Coq Development
Nix is a great package manager that can be used to control various complex environments easily, such as Coq development with ocaml extraction with various dependencies.
Jekyll to create a Portfolio website
Jekyll can be effectively used to publish a static website, even containing multiple pages such as a portfolio.
Emacs as an Email Client
Emacs can be used effectively as an email client to quickly write emails using familiar keybindings, as well as easily linking emails to org files for reminders.