Additives I currently use/have stashed:
MMO
I use it in the gas, sometimes in the oil during a cleanup interval, but only near the end of the OCI(no more than 500-1k miles) and as an aid to oil of choice while transitioning a used car over to synthetic of choice. Though, I suppose if I had sludge problems I'd use the 1QT MMO to the crankcase at the time of a fresh fill for several intervals, different interval drains.
LubroMoly MoS2
I have started using this in my Stratus, but not enough miles to notice a difference, yet. I'm hoping to reduce SOME oil consumption, as this car isn't driven much, while I also want to aid in reducing as much wear as possible. It's only driven 3k miles per year or so lately. Good additive for a great price, like MMO.
LubeGard Bio/Tech Engine Oil Protectant
Okay, I'm going to start calling it LG BT EP for short...
Anyway, I plan to begin using this during my next interval on the Civic. I hope it's like MoS2 but on steroids, in my dreams anyway. Will be having UOA of the Civic's Amsoil SSO without the LG which I'm currently running, and then after the same mileage/oil and LG added for the entire interval will post a UOA again.
Additives I have used in the past:
SeaFoam, occasionally, if a great deal can be found, I will use this in my gas. I did use it in the crankcase before, but would rather use something else like Amsoil Engine Flush if I haven't used anything during the interval for cleanup, but even that is only if my engine is relatively clean already. It won't help sludge etc.
Lucas UCL, my dad always uses it in the gas but I don't. I have before and only sometimes get the placebo 'effect' of better performance. Not sure if it's worth much, but I'm sure it does SOMETHING.
3M to the gas, a certain product of theirs, and afterward I had a ticking sound from one of my fuel injectors on an old car that afterward was totaled for cash, for a different reason.