The Last Message Sent on AIM
One interesting (but probably creepy by todays standards) feature of AIM was that you could put %s anywhere in your profile and AIM would replace it with the screen name of whoever was looking at your profile. An example would be:<p>%s is my best friend!  -> angelc408 is my best friend!<p>This also worked on links you added to your profile so you could make custom links that would send the user's name to your server to make a simple guest book.<p><a href="<a href="http://buddytracker.us/?user=%s">Sign" rel="nofollow">http://buddytracker.us/?user=%s">Sign</a> my guestbook</a><p>I was a sophomore in college when I made a simple guest book service that blew up to over 3 million users in a year. I contemplated dropping out of college my junior year and making it into something more than 20 lines of PHP with a MySQL database. In the end I stayed in school and AIM removed profiles from the desktop client a year later. Dodged a bullet there...