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:%s is my best friend!  -> angelc408 is my best friend!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.Sign" rel="nofollow">http://buddytracker.us/?user=%s">Sign my guestbookI 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...