|Geico's Therapist Sarge|
Other systems, more attuned to a horror trope, handle insanity organically. Call of Cthulhu being probably the most famous - the more you know, the madder you are. Warhammer FRP also contains a system for characters to gain insanity as a byproduct of normal adventuring, gaining them from receiving Critical Hits, failing Fear tests and drinking certain poisons. Disorders once acquired in that system are almost impossible to remove, the character ends up as a quivering mass of psychoses.
In my campaign, I've instituted the following home-brew insanity rules, certain events such as Critical Hits, specific horrendous or chaotic sights - like the cathedral transformed into a giant skull cause the the players to gain Insanity Points. Every time they do, they make a Will save DC = current Insanity Point total. So it becomes progressively harder to pass the check. The difficulty of the check then determines the severity of the disorder.
< 10 - Eccentricity (Always checks for rain when stepping through a door, never sleeps above the ground floor or other behavior the DM finds amusing)
10-20 - Minor disorder (Obsessive/compulsive behavior, Fear of a creature causing the character to be surprised every time they see one or another disorder that places the character at a minor disadvantage)
21-30 - Serious disorder (Paralyzing fear of a creature, schizophrenia or another issue that changes how the character reacts)
30 + Disabling disorder (Severe agoraphobia, catatonia or other issue that prevents normal movement)
Failing the check sets the character's Insanity Point total to zero. Until the next time.
And now I have another list to make of insanity types.
[Ed. Forgot to say this was inspired by The Rusty Battle Axe ]