Hi, welcome onboard.
Most popular sims used on here are:-
GTR2 - for GT racing in various guises/classes. Bit long in the tooth now but still moderately popular.
Race/GTREvo - the successor to GTR2, improved gfx, sounds and feel imo. All sorts of classes available from touring cars, to single seaters to GT's. Interest stirring here bit by bit and some good new content being released for it.
iRacing - you know already, many love it, many just dont see what the fuss is all about, and its pricey (relatively speaking).
GTLegends - been around on here since the ark and still popular, vintage racing classes from 1960's on.
Power & glory - uses GTR2 then with the P&G modpack to create a very good classics racing sim, also popular on here.