It doesn't work with the API, it works with packet sniffing. You need to listen on port 1863 for BYE and IRO commands.... (<= EDIT: this has been changed in recent protocols AFAIK; principle stays the same though)
For as far as I know, there are six programs to do this.
Messenger Discovery and StuffPlug-NG are both programs which aren't dedicated to this feature. This means that they contain more features then showing who has opened and/or closed a conversation with you.
The other programs are dedicated. Thus, this means that they are specificly designed for this feature (and often have more features for this then the non-dedicated plugins)...
An addon for MSN Messenger (by ecko_complex AKA effekt):
1b) An addon for MSN Messenger ("Messenger Discovery" by effekt) :
Big concurrent of StuffPlug-NG.
(Run the installer)
(Not a dedicated addin; It has many more non-related features...)
2a) A plugin for Messenger Plus! ("Notifier" by DJMystic AKA TheBlasphemer):
(Place the DAT and DLL into your plugins directory, start the plugin with command /xnot)
(A new one is in the make, it will be included in "StuffPlug-NG")
2b) A plugin for Messenger Plus! ("StuffPlug-NG" by TheBlasphemer) :
Most used plugin.
(Not a dedicated plugin; It has many more non-related features...)
3) A plugin for Messenger Plus! (uncompiled, by Hah):
(Still alpha example version. You need to compile the source first with VB6.)
4) A plugin for Messenger Plus! ("OpenConvo" by Wouter) :
Most configurable notifier of all.
Old version: http://msghelp.net/attachment.php?pid=219848
(Place the EXE and DLL files in your plugins directory, register the DLL and run install.reg)
New version: http://msghelp.net/showthread.php?tid=28166
(Run the installer)
All the notifier features in these addons/plugins were designed, tested and working in MSN Messenger 6.x.
If you want to know what the plugins exactly do and what the differences are, read the appropiate linked-to thread, don't ask here...
If you have any additions to this list, post them in this thread or PM
SINCE THEN, THE PROTOCOL WHICH THESE NOTIFIERS RELIED UPON HAVE BEEN CHANGED GREATLY SO THEY DO NOT WORK ANYMORE, NO MATTER WHAT PEOPLE CLAIM.
THEY DO NOT WORK ANYMORE, NOT WITH MSN MESSENGER 6, NOT WITH MSN MESSENGER 7, NOR WITH WINDOWS LIVE MESSENGER OR WHATEVER.
(study the protocol before claiming anything else)