LiquidMantis has asked that I start this thread. I will try to make it useful. I don't claim to be the fastest racer or an expert, but I do claim to own almost every one of them. I am only going to address the PC-only sims. There really isn't a PC Sim that is the equivalent to GT or Forza. These are made to experience as long (30 min - 3 hour) races and plenty of qualifying and practice. Only for the serious.
iRacing - There is a debate, but iRacing probably has the best feel of any simulation out there. The physics just feel "right". The latest version, 2.0, was released in August. This release was the debut of their new tire model. Responses to the new model are mixed but I love it. Most the cars still use the old model. The iRacing service is a subscription service. The basic subscription comes with a few cars and tracks but if you want additional cars and tracks, each will cost you anywhere from 12-15 bucks. That may seem pricey, and it is, but the tracks are meticulously created with laser scanners and each car feels very unique. Additionally, other than testing and time trials, there is no single player mode. This is a pure multiplayer simulation. Be prepared to get your ass handed to you as well. Many people make the mistake of stepping into iRacing and buying every single car and track, almost treating it like a very expensive Forza. As fun (and expensive) as that may be, you really need to approach iRacing by attacking one car at a time. Pick a series for the current season and practice, practice, practice. A new track will show up every Monday night for the series. Practice, qualify, and race throughout the week. You can race as many times as you want. The service will average your results and give you a score each week. Also, there is a safety rating system in place which works fairly well. Any incidents in a race, even going off track, will reduce that rating. If you get your rating low enough, you will be prohibited in participation from many series. If you want to tackle iRacing, you probably need to decide that will be your sim of choice. It can be messed with casually, but to get your money's worth, it requires dedication, hours and hours of dedication. Right now, the first two months are free. Support for all FFB wheels on the market is fantastic, by far the best. Triple-screen works without a hitch. We have a GWJ catch-all here. We wanted to get something going but we only got a few races in. Nyxs Optare is hard to beat but pleasant enough, for a Texan The best part of iRacing is that there is always someone to race.
rFactor - This is the best value sim on the market. It uses the gmotor2 engine which is used in GTR2, Race 07, and several more. The gMotor2 engine is one of the easiest things to work with and provided 3 screen support years before others figured it out. After purchasing the base game, there are endless quality tracks and mods available at sites like rFactor Central or No Grip Racing. In my opinion, the best tracks come from http://virtua-lm.com/ and the best cars from Enduracers. It will take a while to get your wheel feeling right. There is a need to play with the options as you change cars. There are several recommended mods out there such as RealFeel to make the experience complete. Single player AI is pretty good. Multiplayer is fantastic but not regulated like iRacing.
Game Stock Car - This is also gMotor2 engine, but heavily modded. It was created by a team that built quite a few quality mods for rFactor. Don't let the name fool you, the focus of this game is Brazilian stock car racing which looks nothing like Nascar. This is road racing at its best. These cars are beasts and you will not find the same tracks (except Interlagos) in any other game. The cockpit motion and sound is fantastic, better than Shift 2 IMHO. I love this simulation. AI is phenomenal.
Race 07 - Sometimes referred to as GTR Evolution, which was an early expansion. More gMotor engine. I prefer the STCC and STCC 2 cars and tracks more than anything else out there. Available through Steam, along with all its expansions. Steering wheel setup seems a bit easier than rFactor, even if it is the same engine. I would love to see a GWJ group that races STCC.
netKar Pro - Arguably the best physics in PC racing simulation. The UI is a bit to wrestle with but much better than it used to be.
Live for Speed - A unique physics engine that feels different than anything else. Once you get used to it, it is hard to go back to anything else. Many folks like it best of all the PC sims. AI is weak but the multiplayer can be fantastic. I have had some epic duels in this one. Well worth the money.