golangy
Libraries for building command-line interfaces: flags, subcommands, config.
Cobra
The framework behind kubectl, gh, and most serious Go CLIs — subcommands, flags, and shell completion, all conventionally structured.
urfave/cli
A simpler alternative to Cobra for CLIs that don't need a nested subcommand tree.
Viper
Configuration from flags, environment variables, and config files with one clear precedence order — usually paired with Cobra.