Try it. Right now, in this page.
This sandbox booted NPSP which is the Nonprofit Success Pack, 26 triggers and 1,503 Apex classes of real, open-source Salesforce code. Fidelic loaded its actual source at boot and parsed every line. Nothing here is mocked or pre-recorded. NPSP © Salesforce.org, BSD-3-Clause. Fidelic is not affiliated with or endorsed by Salesforce.org or Salesforce, Inc.