Many web sites suffer from others directly linking to their image, video and other content. This practice is called often called leeching, hot-linking, or inline-linking and causes wasted bandwidth and increased server load to the victim web site.
Last weekend, I wrote a little ASP.NET module that prevents hot linking. It can be used on IIS5 (Windows 2000/XP), IIS6 (Windows Server 2003), and IIS7 (Windows Vista / Longhorn Server).
It also was a great excuse to talk about IIS and ASP.NET integration history :)
Read more on http://mvolo.com/2006/11/10/stopping-hotlinking-with-iis-and-aspnet.aspx.