brew-cleanup-orphans.zsh

#!/bin/zsh

declare -A visited_formulas

check_formulas() {
    for formula in "$@"; do
    if [[ -z `brew uses --installed $formula` ]] && ! (( ${+visited_formulas[$formula]} )) && [[ $formula != "brew-cask" ]]; then
        read "input?$formula is not depended on by other formulas. Remove? [Y/n] "
        visited_formulas[$formula]=1
        if [[ "$input" == "Y" ]]; then
            brew remove $formula
            check_formulas `brew deps --1 --installed $formula`
        fi
    fi
    done
}

echo "Searching for formulas not depended on by other formulas..."

check_formulas `brew list`

This is great! I use it for ~2 years now. Recently I tweaked it a little bit: upgraded to latest brew CLI behaviour (brew list wants a --formula argument) and added a KEEP_FORMULAS feature. Have a look at https://gist.github.com/cgrothaus/ef9004404e0d168c3d150c409c9d5526 if you like that.