You got the ISI car all really nice (can be found in their homepage):
Nissan GTR GT1
Corvette GT2
Camaro GT3
Nissan 370z
Renaul Megane Trophy
Honda Civic BTCC
Renault Clio Cup
URD GTE Mod
MAK Corp Mazda 787
The old open source LMP1 and LMP2 car that
@Stefan Woudenberg made, which are probably a bit outdated now.
Blancpain Endurance Series Mod (has 8 cars or more)
Apex Modding (SLS, Porsche, Corvette)
The Ferrari 458 Challange a race car version of the road car, but still very low downfroce and a lot of fun
The several Spec Miatas with diffrent stages of power.
There was a V8 Ute mod sometime ago, which was a lot of fun (no idea where that is)
There is also a converted V8 Supercars Mod, but I would wait for the newely build one.
A few things that will come up in the near future (can be found on the ISI forum):
Aston Martin Vantage GT3 (Apex Modding)
BMW Z4 GT3 (Apex Modding)
A full V8 Suoercars 2014 Mod featuring Volvo, Ford, Holden and Nissan
An LMP1 & LMP2 Mod by URD that should feature 5 to 10 prototypes some are already used in an endurance leauge.
An NGTC (BTCC mod) including a Mercedes, Audi etc.
Hopefully a BOP Mod that equalizes the ISI GT1 to GT3 cars and maybe the Apex Modding cars.
I also found a BTCC Mod for a BMW and a Cruze, but not sure, if it is on the ISI forum
That are quite a lot of cars. Unfortunately there is no proper central site as things like rfactorcentral seem to deny the existence of rF2. The thing that is mostly missing are more prototypes and GT1 and GT4.
Maybe someone has more to add.
Good idea:
Maybe we can setup an extra thread here in the Modding section, which lists all cars and where can you get them (not sure if we would be allowed to link there). It is hard to get a good overview about all the content. For tracks it is even more difficult.