// Get the node from the current path.
$node = \Drupal::routeMatch()->getParameter('node');

// Get current path.
$path = \Drupal::service('path.current')->getPath();

// Get path arguments (from path above).
$path_args = explode('/', $path);

// Get the current route.
$route_name = \Drupal::service('current_route_match')->getRouteName();

// Get the query parameter from a GET request.
$name = \Drupal::request()->query->get('name');

// Get the parameter from a POST request.
$name = \Drupal::request()->request->get('name');

// Get the host (ex: www.google.com).
$host = \Drupal::request()->getHost();

// Redirect.
use Symfony\Component\HttpFoundation\RedirectResponse;
new RedirectResponse(\Drupal::url($route_name));

// Add t() to classes (services, controllers, etc)
use Drupal\Core\StringTranslation\StringTranslationTrait;
class MyClass {
  use StringTranslationTrait;
}