passing URL to Webform
On you link display something like this :

$current_path = current_path();
$current_path_alias = drupal_get_path_alias($current_path);    
print url('my-webform', array('query' =&gt; array('from' =&gt; $current_path_alias )));


If you are on content/mynode, the link will display my-webform?from=content/mynode.
If you want just the unaliased path skip the use of dupal_get_path_alias()

$current_path = current_path(); 
print url('my-webform', array('query' =&gt; array('from' =&gt; $current_path)));


If you are on node/18, the link will display my-webform?from=node/18.