A quick google search for ‘related posts widget’ should sort you out. Here’s one…
http://wordpress.org/extend/plugins/yet-another-related-posts-plugin/
You’ll need to create an extra sidebar module to use the widget only in single.php. Here’s the easy way….
Add this to your functions.php file.
Code:
if ( function_exists(‘register_sidebar’) )
register_sidebar(array(‘name’=>’related’,
‘before_widget’ => ‘
‘,
‘after_widget’ => ‘
‘,
‘before_title’ => ‘
‘,
));
then add this to single.php where you want the widget to appear.
Choose ‘related’ from the drop down menu in Themes->Appearance->Widgets and add the ‘related posts’ widget to that sidebar module.
Sorted!