Pokefight is a free online/LAN pokemon battle simulation app.
Download pokefight-client.zip in the latest release. Unzip it, and double-click on pokefight-client.exe
One of the players should launch the server (i.e. download pokefight-server.zip, unzip it, and double-click on pokefight-server.exe). Enter the port you want when you're prompted to. (e.g enter 7777)
Then all the client that want to play must enter the ip/hostname of the server. And then enter the port.