Here's a new and improved version of Link-a-word:

<?php
/**
* This file implements the LinkWord plugin for b2evolution
*
* Automatic Linking of key words
*
* This hack is based ENTIRELY on Isaac's {@link http://isaacschlueter.com/} groovy
* AutoAcro plugin, and was converted to LinkWord by EdB {@link http://wonderwinds.com/}
*
* b2evolution - {@link http://b2evolution.net/}
* Released under GNU GPL License - {@link http://b2evolution.net/about/license.html}
* @copyright b2evolution (c)2003-2004 by Francois PLANQUE - (@link http://fplanque.net/)
*
* @package plugins
* @subpackage renderers
*/
if( !defined('DB_USER') ) die( 'Please, do not access this page directly.' );

require_once dirname(__FILE__).'/../renderer.class.php';

class linkword_Rendererplugin extends RendererPlugin
{
var $code = 'LinkWord';
var $name = 'Link-A-Word';
var $priority = 40;
var $apply_when = 'opt-out';
var $apply_to_html = true;
var $apply_to_xml = true;
var $short_desc;
var $long_desc;

/**
* LinkWord array - this is where you set up your keywords and associated URLs
*/
var $search = array();
var $replace = array();

/**
* Constructor
* {@internal linkword_Rendererplugin::linkword_Rendererplugin(-)}}
*/
function linkword_Rendererplugin()
{
$filename = dirname(__FILE__).'/../../skins/custom/wordlinks.txt';
$linkwords = fopen($filename,"r");

while(!feof($linkwords))
{
$values = preg_split("/:/",rtrim(fgets($linkwords)),3,PREG_SPLIT_NO_EMPTY);
$this->link_search[] = '/\b('.$values[0].')\b/i';
$this->link_replace[] = '<a title="'.$values[1].'" href="http://'.$values[2].'">$1</a>';
}

fclose($linkwords);
$this->short_desc = T_('Automagically link to sites you like');
$this->long_desc = T_('Words added to the plugin array will automatically be linked to the designated URL.

/**
* Perform rendering
* {@internal linkword_Rendererplugin::render(-)}}
*
* @param string content to render (by reference) / rendered content
* @param string Output format, see {@link format_to_output()}
* @return boolean true if we can render something for the required output format
*/
function render( & $content, $format )
{
if( ! parent::render( $content, $format ) )
{ // We cannot render the required format
return false;
}

$content = preg_replace( $this->link_search, $this->link_replace, $content,1 );
$content = preg_replace( '#(<[^<>]*)(<[A-Za-z]+[^>]*>)(.*)(</[A-Za-z]+>)([^<>]*>)#', '$1$3$5', $content);
return true;
}
}

// Register the plugin:
$this->register( new linkword_Rendererplugin() );

?>

Each line of the wordlinks file should have this format:

keyword:tooltip:link

A quick example:

b2evo|blog:Everyone's favourite:www.b2evolution.net
slashdot:Geek news:slashdot.org

The first token allows to use OR expressions and you should bear in mind that you may not include the separator : in any of the strings, unless you change the separator to something else.

Also, you should change the file path to whatever suits your needs.