Here is a nice little trick I implemented on this site. Are you tired of looking at the who's online list and seeing unknown action next to one of your users or guests.

Well check this out. A real quick way to do it is to make a change in your Who.php file that is located in your Sources directory. Here is the edit, thanks to Kindred over at SMF:
In Who.php Find
// Unlisted or unknown action.
else
$data[$k] = $txt['who_unknown'];
Replace with:
// Unlisted or unknown action.
else
// $data[$k] = $txt['who_unknown'];
$data[$k] = 'Viewing the <a href="' . $scripturl . '?action=' . $actions['action'] . '">' . $actions['action'] . '</a>';
There is a bit more at this thread over at SMF. He is also working on sub actions as well I think. Hopefully that goes somewhere as it would be real nice to have:
http://www.simplemachines.org/community/index.php?topic=467136.0