Here's a basic working example plugin, that works in WordPress 2.8 and above.

Just add a folder 'user-column' to the plugins folder, put a file 'user-column.php' into this folder and paste the following code into the file:

<?php 

/*

Plugin Name: Custom User Column

Version: 0.7

Plugin URI: http://www.wpquestions.com/question/show/id/74

Description: &lt;strong&gt;WordPress 2.8+ only.&lt;/strong&gt; Display Custom User Fields on Edit Users Screen 

Author: Oliver Schlöbe

Author URI: http://wpseek.com/

*/



function imel_column_userfield( $defaults ) {

	$defaults['imel-usercolumn-userfield'] = __('Age', 'user-column');

	return $defaults;

}



function imel_custom_column_userfield($value, $column_name, $id) {

	if( $column_name == 'imel-usercolumn-userfield' ) {

		return get_usermeta($id, 'age');

	}

}



add_action('manage_users_custom_column', 'imel_custom_column_userfield', 15, 3);

add_filter('manage_users_columns', 'imel_column_userfield', 15, 1);

?>

This adds a column "Age" to the users liste page displaying a custom user meta key 'age'.

Alter as you like. :)