Tracker tracks recent posts for users and is a core module switched off by default. I might use this if I add forums.
Tracker has the disadvantage of requiring the Comments module even if you do not use comments. Tracker Lite is a replacement for Tracker and does not need the Comments module.
Tracker 2 is an add on module offering the same function as Tracker plus claiming faster performance.
Extended Tracker is a Tracker replacement offering extra features but is still in development for Drupal 6 at a time when all the popular modules are in conversion for Drupal 7.