Project CARS is going to be a pay once type thing. Other sims include:
Richard Burns Rally - old but still the best rally sim, get RSRBR 2013 for it.
Race 07 - very cheap and there are a number of add-on packs. I recommend waiting for the Steam summer sale (either this month or next, I think) as it always gets discounted by 75%.
GTR2 and GT Legends - also on Steam, same warning as Race 07. GTR2 is FIA GT cars, GT Legends is historic racing cars from the Austin Mini Cooper S to Porsche 917K.
Simraceway - free to play, you get access to all circuits and I think three cars for free, the rest you can either buy with real money or in-game credits earned at a rate of 5 per lap.
netKar Pro - limited content but an excellent single seat open wheel simulator. Also gets you access to the Assetto Corsa tech demo (one car and one course).
GP Legends: old (from 1998) and a little tricky to get running, but astonishingly good even by today's standards. Mods pretty much required.
Game Stock Car and Formula Truck: I don't have either of these, but I've heard good things.
Dirt: old, apparently not supported by Windows 7 but a lot of people (myself included) have had no trouble running this. A better sim than either of the sequels but not amazingly realistic...
WRC 3: probably the second best rally sim. Rally Trophy might be better but I've not tried it.
NFS: Shift, Shift 2 Unleashed, Grid, Grid 2: simcade stuff, might be worth picking up for cheap, I don't know.
Personally I think you can't go wrong with Race 07, it's something like £2.50 right now and you get a lot of content in the base game, then when you add the DLC and expansions (GTR Evo, Race On, WTCC 2010, STCC, STCC 2, GT Power, Retro and... I know there's at least one I'm forgetting) you end up with a LOT of stuff. If that's not good enough it supports mods too, though the inconsistency in quality generally puts me off them. The AE86 Club mod is pretty good though, as is the Red Pig mod (300 SEL).