http://mtasa.com/agent/
If this appears in your web server logs, it means either:
* An MTA:SA game server has made a request for one of your web pages.
or
* An MTA:SA game client has been told to request one of your web pages by the game server it is connected to.
Either way, the agent string should contain a port number which you can use with the supplied MTA:SA server IP address to find out the original source of the request.
If you have any abuse complaints, please take it up with the owner of the MTA:SA server.