Although I can't recall any really violent games, and i've not written one myself there are aspects from some stuff i've done which if combined would add to the atmosphere.
I always wanted to write a game where the blood didn't just land on the floor, but splattered onto the players camera view and slowly dripped down... I did this effect ones on one of my Amiga versions of Blood Bowl, but those where early days and it didn't look too great.
I did particle blood effect in Dangerously Hidden, but that was before I understood how fogging affected the particle system so isn't great. My Gettin Shot Up game features no actual blood and guts, but if a character dies near you they say some rather amusing lines such as 'Oh dear, I appear to have lost half my face... urgh' etc.
Other than that, doing some 3d object plains with heart and lung textures on that come flying out of the body as it dies should do the trick.
For real effect though it isn't so much the special effects that make a game seem violent, but the weapon you use to kill the enemies. Take traditional household objects and show them in an application that isn't so innocent, for extra impact use items we normally consider dainty, such as flower stems being used as a garrot...
Pneumatic Dryll, Outrageous epic cleric of EQ/Xev
God made the world in 7 days, but we're still waiting for the patch.
