It is easy on unix to create ad blocking for the web using
/etc/hosts file to redirect certain domains to localhost.  
For example,
127.0.0.1 doubleclick.net

There are a few reasons I suggest adding this to the main 
documentation.

1. it is very easy to do (suitable for beginners)
2. it teaches principles of how unix works (dns, hosts file)
3. it is incredibly!! useful, especially for netbsd-as-home-routers

I think this would fit well within chapter 25.

There are several web pages out there that detail instructions
on how to do this (though they are rarely detailed on the unix
side or netbsd), I'll refer to them here.
http://www.ecst.csuchico.edu/~atman/spam/adblock.shtml
http://pgl.yoyo.org/adservers/

Maybe this thread will inspire someone to create and/or pkgsrc 
a quickie perl daemon to periodically sychronize the 
ad block portion of a hosts file.  That would be a neat hack..

--JC