Not sure what exactly you're after, but I debug plugins in a couple of different ways. One really easy one is to echo out the value of something if that is all I need. Like this:

if( is_logged_in() ) {
echo 'parameter whatever is: '.$whatever.'<br />';
}

It breaks the page because it gets all mad about output starting before something that is supposed to be first, but so what: it's development and I'm the only one seeing it. You can get really fancy with it if you like and dump an array with this:

echo '<pre>';
print_r( $your_array );
echo '</pre>';

Even cooler is to use the message maker thingie that b2evolution provides, like this:

$this-msg( '<pre>'.var_export( $whatever, true ).'</pre>', 'note' );

Hope it helps a wee bit!

PHP IDE that included a debugger?

I'm using notepad++ with php xdebug extension
http://amiworks.co.in/talk/debugging-php-using-xdebug-and-notepad-part-i/
http://www.ourwikicommunity.com/wiki/How_to_Setup_BDGp_debugger_in_Notepad%2B%2B_to_Debug_PHP
It's very usefull.

I use Text Mate with all of it's included bundles.

It corrects me relentlessly

I just bookmarked the first link slamp provided so I can make my notepad++ be reason enough to re-install wamp :)

If you are not using MSWin, try [1] Bluefish and [2] Quantra+
If you are using MSWin, try [1] jEdit and [2] EmEditor

Usually you'll have to find the appropriate plugins, but if I read correctly, Bluefish and Quantra+ provides it as built-in.