More and more, drupal site builders have to make choices that they shouldn't if it comes to "choosing which contrib to use". We often can choose for an API - which is great - or choose for a ready-to-go module.
Heartbeat is an API that allows logging activity messages to the database. These logged messages are listed up to form a heartbeat stream. There are three built-in streams but developers can register their own.
This page holds links to documentation about the module "heartbeat" for drupal 6. From here you will find how to install the module, how to use it and how to implement its hooks.
I intend to write a module to integrate Hyves within drupal sites. Hyves is a facebook alike community website that offers an nice api, much like facebook does. Hyves is used by users in the Netherlands.
User points is module that gives a user points when performing an action. This module has a couple of contributes that give the api the specific desired functionality.