iRacing is a PC exclusive, subscription based, online racing service offering racing against human, but no AI, competitors in various racing series including three Nascar series, Formula One, Indy Car, Australian V8 Supercars and many others. I myself am subscribed and can honestly say it is the most realistic and enjoyable racing simulation I have ever played, however is very costly.
After subscribing you will be provided with 5 cars and 9 tracks to race. Every track and car beyond this will need to be purchased at a price of $11.95 to $14.95. Subscriptions cost between $7.46 to $12.00 per month depending on duration.
Additional information can be found at iracing.com. If you are willing to pay its costs iRacing is a very rewarding experience however Nascar 2003 remains one of the best racing simulators of all time and can be found for a much lower price.