For developers who want to see code in action, the Examples for Developers project is an essential download. It provides a suite of sub-modules demonstrating how to use core Drupal APIs, such as the Form API, AJAX, and Cron. Examples for Developers | Drupal.org
The Drupal community has shifted toward "recipes" and "starter kits" that allow you to launch fully functional sites in minutes. drupal sample site download
Commerce Kickstart is the gold standard for online stores. It includes an installer that lets you choose between a blank site or a full demo store for evaluation. For developers who want to see code in
If you have a specific industry in mind, several pre-built site templates (formerly known as distributions) offer a complete experience out of the box: such as the Form API