Tools/Features/Solutions/Webparts that I've found very helpful

Document Rating
Limited Access Report
My Alerts
Office File Types
Print List (Part of SharePoint 2007 Features)
Site Information
SP Client Utilities
AD Tools for SharePoint
Quick Access Site Settings (see also my post)
Log File Viewer (for SharePoint Central Administration)
Custom Applications Pages (requires free registration)
SharePoint Site Browser (requires free registration)

A lot more free WSS 3.0/MOSS 2007 products at wssv3faq.mindsharp.com