Content Manager went from a mod to a "must" in Assetto Corsa. Makes everything much more simple.
The only other one I can think of is SOL, but that's depending on the PC. Crew Chief is also a nice addition, not only to AC, but pretty much any other sim and some other racing games
Yeah at this point I feel weird not using Crew Chief on AC or ACC. It guves me more information that the regular spotter doesn't. Only issue I ever had is that sometimes it would tell me info such as "you are locking your tires on X corner" and I might not know which corner is that. Personally I would have preferred for it to say the corner number instead of the name, but that's just me.