Frog CMS

with something like that in your foreach loop:

<a href="javascript:ajaxpage('<?php echo $page->url(); ?>', 'leftcolumn');"><?php echo $page->title(); ?></a>

<ul>
<?php foreach($this->find('/')->children() as $menu): ?>
  <li><?=$menu->link($menu->title,"onclick=\"ajaxpage('/".$menu->url."', 'leftcolumn'); return false;\""); ?></li>
<?php endforeach; ?>
</ul>

This should work.
Also mention that in this example links stay work even if javascript is turned off in browser.

A better method (graceful degration) would be to have the common foreach loop:

<ul>
<?php foreach($this->find('/')->children() as $menu): ?>
  <li><?php echo $menu->link(); ?></li>
<?php endforeach; ?>
</ul>

and then later add a javascript click()-event all links.
When a user does not have JS enabled, he can navigate anyway.

By the way: See my plugin for only loading the page content via ajax instead of the whole page.

Yep, but only a really little bit with jQuery:

$('a').click(function(){
  ajaxpage($(this).attr('href'), 'leftcolumn');
});