I've been working on a user notifications module (and underlying API module) for Drupal.

This is initially needed for for use with my classified ads module but it can be used by any module that needs a well-defined method of alerting users to various events.

Features:

• Per-user notification lists
• Optional email notification sent to user (per-user opt-in)
• Notifications administration menu (admin, or per-user)
• Notification info blocks
• Automatic expiration of old notifications