Another excellent sim is Papyrus' NASCAR Racing 2. This was written after ICR2, and has much superior menus and other features. Its most important improvement is networked multiplayer mode. With this sim, you can race with up to 8 live opponents on a LAN, or via the Internet. If ICR2 had this feature, and N2's menu system, it would be unbeatable. Unfortunately, I don't enjoy driving stock cars as much as I like driving Indycars, so N2 doesn't get too much action on my computer.
As with ICR2, you need a Rendition-based 3D card to get the best visual details and optimum frame rate in N2.
There is a great enhancement to N2, known as The Pits Touring Car Championship, available from The Pits. This replaces the NASCAR cars with cars from the British and North American Touring Car Championships, such as BMWs, SAABs, and Hondas. It also changes the engine sounds, and modifies the vehicle dynamics somewhat, making the cars lighter and more nimble, though less powerful. It is a great example of how ingenious reverse-engineering and rewriting can enhance an already excellent sim.
Papyrus recently introduced their own enhancement package for N2, called the Busch Grand National Series Expansion Pack. This includes some of the newer tracks like California Speedway, Gateway International, and Texas.Motor Speedway, as well as some fantasy tracks. Stay tuned for a review, now that I've got a copy in my hot, sweaty little hands.