I keep handing this plugin out to people on request so I might aswell post it here to the forums, atleast then I can just link them to it.
You can configure what messages it logs in "messages.txt".
There is some bugs in AMXModX 0.16 Engine module that stop this from working perfectly. The ID of the player the message was sent to isn't detected, and on map change the server will crash. These problems are fixed for 0.20.