VROC or WinVROC was the first, widely used GPL online client. Here's a quote from the original website [http://www.vroc.net/]:
The Virtual Racers' Online Connection, or VROC (pronounced VEE-rock), is intended to help you find others who would like to race GPL over the Internet. Here you'll find people hosting GPL, and you can host your own race if you wish.
It's still popular and you can download it from here: [http://www.vroc.net/]