You can knock foes off their feet with your shield while charging.

<php>echo “<b>Prerequisites:</b>”;</php> Improved Shield Bash, BAB +3
<php>echo “<b>Benefit:</b>”;</php> If you hit your opponent with a shield as part of a charge action, in addition to dealing damage normally, you may make a free trip attack without provoking an attack of opportunity. If you lose, the defender does not get to try and trip you in return.