Make Bash utilities usable in Fish shell.
Bass makes it easy to use utilities written for Bash in fish shell.

Regular bash scripts can be used in fish shell just as scripts written in any language with proper shebang or explicitly using the interpreter (i.e. using bash script.sh). However, many utilities, such as virtualenv, modify the shell environment and need to be sourced, and therefore cannot be used in fish. Sometimes, counterparts (such as the excellent virtualfish) are created, but that's often not the case.

A nix and nix-shell wrapper for shells other than bash.

nix develop and nix-shell use bash as the default shell, so nix-your-shell prints shell snippets you can source to use the shell you prefer inside of Nix shells.

Powerline is a statusline plugin for vim, and provides statuslines and prompts for several other applications, including zsh, bash, fish, tmux, IPython, Awesome, i3 and Qtile.

Powerline @ GitHub

The minimal, blazing-fast, and infinitely customizable prompt for any shell!

Starship Prompt @ GitHub

Cod is a completion daemon for {bash,fish,zsh}.
It detects usage of --help commands parses their output and generates auto-completions for your shell.

A package manager for shell scripts and functions.

Basher allows you to quickly install shell packages directly from github (or other sites). Instead of looking for specific install instructions for each package and messing with your path, basher will create a central location for all packages and manage their binaries for you.