Is it a way to get know if more than one user edit the same node? I'm looking for the way to inform site editors if the other editor is trying to edit the same node. In other words I need functionality that is similar to the Checkout module. But it is no longer maintained and I don't like to lock anything, just show a popup message.

Do you think that it is possible? I didn't find any hooks in Drupal API that could do that. 

And the last thing - this is Drupal 6
This is already built-in in Drupal 6 (and 7), in the function node_validate.

The relevant piece of code is reproduced below:

if (isset($node-&gt;nid) &amp;&amp; (node_last_changed($node-&gt;nid) &gt; $node-&gt;changed)) {
  form_set_error('changed', t('This content has been modified by another user, changes cannot be saved.'));
}


It doesn't lock anything, but it shows the second editor a message, and prevents the second editor from saving and thereby overwriting what the first has done.

The second editor can then copy-paste his/her efforts to Notepad or other temporary buffer, reload a fresh version of the node (i.e. a version that contains the first editor edits), add his stuff again, and save.