Wordpress: hook in the tag-list table

wp-manage_category_custom_column.php

add_filter( 'manage_category_custom_column','myTestC');

function myTestC($content){
  return $content; 
}

add_filter( "manage_category_custom_column", array( &$this, 'myTestC' ), 10, 2 );

function myTestC($column_name, $id){
var_dump($column_name, $id);
#return $content;
}

Als Erläuterung, aus dem Core:
apply_filters( "manage_{$screen->taxonomy}_custom_column", '', $column_name, $tag->term_id );
Das ist dein Hook, category ist ja nur eine der Taxonomien, und dann kommen die beiden Werte, name der Spalte und Id

add_filter( "manage_category_custom_column", 'myTestC', 10, 2 );

function myTestC($column_name, $id){
var_dump($column_name, $id);

return $content;

}

add_action( 'manage_edit-category_columns', 'myTest');
function myTest($content){
$content['test'] = 'hallo';
return $content;
}

add_filter( "manage_category_custom_column", 'myTestC', 10, 3 );

function myTestC($column_name, $id, $der){
var_dump($column_name, $id, $der);

return $content;

}

//create a new row
add_action( 'manage_edit-category_columns', 'myTest');
function myTest($content){
$content['test'] = 'hallo';
return $content;
}

//fill the row with content
add_filter( "manage_category_custom_column", 'myTestC', 10, 3 );
function myTestC($column_name, $columid, $termId){
var_dump($column_name, $id, $termId);
}

this works, with WP Codex ;)
//create a new row
add_action( 'manage_edit-category_columns', 'my_test' );
function my_test( $column ) {
$column['test'] = 'hallo';
return $column;
}

//fill the row with content
add_filter( 'manage_category_custom_column', 'my_test_c', 10, 3 );
function my_test_c( $foo, $column_name, $term_id ) {
var_dump( $column_name, $term_id );
}