For those who want a Sanity mechanic in their RuneQuest game, there are several options to choose from.
"Fear and Madness" - Necromancy for RuneQuest II from Mongoose
"Sanity" - Renaissance Deluxe from Cakebread & Walton
"Psychological Stability" - Luther Arkwright for RuneQuest6 from Design Mechanism
And of course the first RPG to feature a Sanity mechanic - Call of Cthulhu. From my perspective using CoC's Sanity system has an important advantage - every creature from the "Cthulhu Mythos" is already stated for it.
Here's my tweak on the CoC Sanity system. Link to the pdf - Sanity
Characters have a number of Sanity Points equal to their POW x 5 ½. The actual Sanity check is made with a Willpower roll.
Since beginning characters will typically begin with a high Sanity rating (POW 13 = Sanity 72) and relatively low Willpower rating (POW 13 = Willpower 26), this has the effect of new characters being well-adjusted, but somewhat soft and vulnerable to psychological trauma. As a character becomes more experienced he tends to become both mentally hardened and scarred by his travails, accordingly his Willpower increases and his Sanity erodes.