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 <META NAME="ROBOTS" CONTENT="NOHTMLINDEX"> to be <META NAME="ROBOTS" CONTENT="NOHTMLINDEX,NOINDEX">
• WebPartZone Adapter
  Replaces the <Table> tag with a <DIV> tag in Webpart zones
• Web Slice Control adapter for MOSS 2007
  The following example will demonstrate how to easily implement a simple control adapter which will render the required html code for turning any web part on your server into a live web slice.
• zevenseas Feature Blocker
  After writing the code to block only the publishing feature on the managefeature.aspx page, I was thinking of making it more generic. In such a way that a SharePoint administrator could define in the Central Administration which features are not to be activated or deactivated by the users per web application.