Case Tracker

Case Tracker enables teams to track outstanding cases which need resolution. This module was inspired by the issue handling in project module, but unlike Project module, which is specific to software development, Case Tracker was designed to be a generic issue tracker.

Case Tracker is usable right out of the box as a basic ticketing system, it's default UI is minimal but feature complete. The module was designed to be extended by programmers and site builders to meet complex needs, and so tries to avoid making too many assumptions about all the ways it can be used.

Drupal 8

8.x-1.x - Is not complete but is under development at this time. A new fields based approach allowing cases to be attatched to not just projects, but any desired entity is being attempted.

Drupal 7

• 7.x-2.x - it is an Entity based approach(#1653184: Case Tracker 7.x-2.x - a new approach, with entities) and is currently abandoned. Work on different approaches will begin with the 8.x-1.x branch.
• 7.x-1.x - has the content types for Project and Cases working without problems. The Notifications, Actions and View integration that needs to be tested.

Drupal 6

Is fully featured and stable, but we are heading to Drupal 8, so it's a bit outdated.

For other similar tools in Drupal, this comparison may be useful