WhosOnline was an application that would scour the offline buddies of an AIM screename and attempt to discover those which were invisible. This software is long since non-working.

WhosOnline used a simple bug in AIM that would allow the presence information of someone invisible to be available through their profile.  Simply requesting the profile and examining the response would allow one to determine whether someone was invisible or offline.

AOL has long since patched this bug, and WhosOnline is no more.  There is no available download–but it wouldn’t discover invisible buddies anyway.  This website is linked to, and I receive many requests for this application, but it simply doesn’t exist anymore.