RFactor has loads of add on cars and tracks, if you like new content then its the best racing game, off course some add ons are better than others, but a good few of them are really high quality, such as the Megane Trophy, Porsche Carerra Cup, Lupo's (yeah sounds stupid but great close racing online) and there is a good few others that can waste your time away, plus 100's of tracks
There is add on cars/tracks for GTR/GTL as well but the cars seem a little less varied, Netkar Pro I would steer clear of just now, as its only really meant for online racing as it has no AI then its not worth it as the online coding is pretty damn horrible and you will struggle to get any enjoyment out of it, but they are working on it so maybe it will be worth a look in a few more months.
GTR2 is on it's way and is looking quite impressive, to be honest I thought GTR was the best sim ever, really did feel like you were in a race car, but it was a bit dodgy online, if they sort that out then it will be a clear winner IMO.