ZDoom

For hitscans it may be useful. But what about projectiles? As I know, using facetarget won't turn actor to the projectile that hit him.
For hitscans It's simple, but projectiles... I was thinking about it, and I think the most realystic way is to thrust actors using projectiles' pitch, not the angle between projectile and the actor as I said before (this will be looking like billiard)
Sorry please, my English sucks=) I hope, you understood...

Sure you can, just use A_FaceTarget.

By the way, I can't :P Currently there is no way to get coordinates of the thing that hit you using acs. I can do constant or random z thrusting, but I can't do thrusting that will depend on angle between the actors.

You can do this with custom pain states and Z thrusting, by the way

Normally an Actor that was hit by projectile or bullet gaines X and Y momentum, ignoring vertical angle between the actors and projectile. With ThrustZ flag a projectile will also thrust actor by Z axis.
Or must that be a console/compatibility option affecting all hitscans and projectiles? I'd like to play doom.wad with it. You can have some real fun with spidermastermind - set low gravity, try to jump on it, and he'll push you upwards with his chaingun :P I also need this feature for realystic physics in my project. For now the only way to make actor gain z momentum is to use a projectile with radiusdamage.