If you are a web professional using Google Adsense and other advertisement methods, you may be supplying content and services to people that have your advertisements blocked.  The least these companies could do is provide web site owners a method to deny services to those that are using their software.

Here is a free "ad blocker" – blocker.  It will direct people to another page if they are running a blocker.  If anyone finds more "ad blocker" blocker programs which are free, let me know and I will add them to this post.

http://www.blue.lu/other/blocker/

Another good site for blocking - blockers is:

http://www.adblock.org/

It appears that Norton Internet Security 2003/2004 can deface your site and block your advertisements without the user even knowing they are being blocked.  Not only do they default to block ads without the user's knowledge, they allow other ads (that I can only think they must have worked out a deal with prior) go through without blocking.  That is at the bottom of the page there in the AdBlock.org site (link below).  Seems Apple is one that you will always see their ads even though Norton detects them.

This can be a major issue with sites.  For a more information see the site:

http://marketing.about.com/cs/internetmarketing/a/norton2004.htm - Lots of links on this page to even more information.

and

http://http://www.adblock.org/article_nis_block_list.html