RFactor is, arguably, one of the very best, if not the best, platform for car racing sims. ISImotor 2.0 (gMotor) debuted in 2005 with rFactor and since then has been widely used in many different racing games and racing simulations on the PC (SimBin's, Reiza's, etc).
IRacing, NetKar Pro and a few others, although very very good sims, are not, or just don't feel, as "complete" as ISI's title.


RFactor Mods in this video:

DRM Revival 2.0b
Pixim Cars v2
Blancpain Endurance Series 2012
F1 2012 by Sandrox
Historic GT & Touring cars v1.95
Formula Armaroli
F1 1975 IPE v1.2
Legends Cars
Endurance Series by Enduracers SP2
F1 S-R 1994 L.E v1.2
Super TC2000
Corvette C6 2012 by Niels Heusinkeld
37 GP mod

Tracks:
Nordschleife Evo conversion; Velopark (from GSC2012 Trackpack ); Londrina (from GSC2012 TP); Mid-Ohio (by Virtua_LM); Rouen by (Virtua_LM); Spa Francorchamps 1988 (by Carrera4); Silverstone 79; Mills Metropark; Interlagos (from GSC2012 TP); Kyalami 93 (from F1 SR TP); Potrero de los Funes; Fuji (by Virtua_LM); LeMans (by Virtua_LM).


Specs: phenom II x4 965@ 3.40ghz, 8gb ram, HD6870, Logitech G27


Check out: http://rfactor.net
http://www.rfactorcentral.com for mods, tracks, etc

http://www.enduracers.com/images/endu... "Forza 2" logo on Peugeot 908's dashboard.