Laravel 5.8 to 8.0 and above
Simply pass it as a paramter:
php artisan serve --port=8080
You may also bind to a specific host by:
php artisan serve --host=0.0.0.0 --port=8080
Or (for Laravel 6+) you can provide defaults by setting SERVER_PORT and SERVER_HOST in your .env file. You might need to do php artisan cache: clear as well. (thanks @mohd-samgan-khan)
And if you want to run it on port 80, you probably need to sudo.