The most-used one from a brief survey
seems a little bit lighter-weight, and the second most popular
An implementation of docopt in the Go programming language.
ff stands for flags-first, and provides an opinionated way to populate a flag.FlagSet with configuration data from the environment. By default, it parses only from the command line, but you can enable parsing from environment variables (lower priority) and/or a configuration file (lowest priority).
Kong aims to support arbitrarily complex command-line structures with as little developer effort as possible.
To achieve that, command-lines are expressed as Go types, with the structure and tags directing how the command line is mapped onto the struct.