I've created a simple script to track recent user activity on SMF. It's nothing fancy, but if you want to find out what people have been up to (useful for keeping tabs on potential trouble) you may find this handy. You can download the files from here: http://army.ca/deleteme/tracker.zip
Installation is simple, just drop it in to your SMF directory (same one that contains SSI.php), then browse to tracker.php.
You can enter the username, display name or user ID# for a user to track. Due to the nature of the info provided, currently only forum admins are allowed to run the script.
I've included a couple screen shots to show the general functionality.
Update: Now shows moderator actions as well.
Update [10 July 2009]: A new version for SMF 2 is now available at the above link.