Many Drupal modules integrate with the Token module and it is used in almost every Drupal install for defining path aliases and even custom breadcrumbs.  Its so popular that in the upcoming Drupal 7 release token is integrated into core.  

If you're developing your own custom module why not integrate with token and allow your users more flexibility with what they can do with your content? With two simple functions you can easily integrate your module with Token.

function mymodulename_token_values($type, $object = NULL) {
	if ($type == 'node') {
		if ($object->type=="NODETYPE") {
			$values["token-name1"] = check_plain($object->var1;
			$values["token-name2"] = check_plain($object->var2);
		}		
    return $values;
  }
}

In the above function $type can also be $user or $global. After declaring what the values are for your token you need to define descriptive text for them:

function mymodulename_token_list($type = 'all') {
  if ($type == 'node' || $type =='all') {
    $tokens['cat']['token-name1']            = t('A description of token name 1');
    $tokens['cat']['token-name2']          = t('Token name 2's description');
    return $tokens;
  }
}

That's all their is to it. The documentation page on drupal.org gives good examples of this as well as points you to token_node.inc inside the Token module for more information.