Control Adapters
With a Control Adapter you can customize the rendering of an attached control. Certain control methods can be overridden with custom code, to make the control behave differently.
Some Control Adapters for SharePoint, which already exist:
Adjusting the MOSS ROBOTS meta tag for 3rd party search engines – using a Control Adapter
Modifies to be WebPartZone Adapter
Replaces the tag with a tag in Webpart zones Web Slice Control adapter for MOSS 2007