My default Privoxy (a great Privacy-enhancing/ad blocking proxy) setup was blocking the on-demand section of New Zealand's TV3 sites (both 3news.co.nz and tv3.co.nz).

Here are the minimum rules that let the videos through:

{ -block }  
.doubleclick.net/[0-9]+/DartShell.*\.swf  
.doubleclick.net/pfadx/DARTSHELLCONFIGXML  
.doubleclick.net/crossdomain.xml  
.doubleclick.net/pfadx/3news.co.nz  
.doubleclick.net/pfadx/www.tv3.co.nz  

If you are looking to unblock a different site, here's how you can do it:

  1. set debug 1 in /etc/privoxy/config
  2. restart privoxy (sudo /etc/init.d/privoxy restart on Debian)
  3. watch the logfile for any blocked contents:

    tail -f /var/log/privoxy/logfile | grep Blocked