Grand Prix Legends and Viper Racing have the best FF implementations to date, but I'm pretty sure their demos do not have FF enabled. Also, the .ini file in Grand Prix must be modified to enable FF but it is a one line edit.
Both games can be found for cheap. Viper is $10 at many places, including Amazon. Grand Prix Legends can be found for a whole $4 at many Office Depots, and is arguably the most accurate driving sim, and certainly the most awesome (Physics engine runs at 288Hz, individual car parts are modeled (most sims just model the whole car based on its C.O.G.) ever created.
Nascar Heat, Nascar 3, Nascar Legends, the NFS3/4/5 series, F1Racing Simulation, Monaco Grand Prix Racing (aka F1Racing Sim Pt.2), F1 Championship, Dirt Track Racing - Sprint Cars are others that have FF, but IMHO not as well done.