Having seen the Stalker film in the cinema last year, and just finished reading the Roadside Picnic book, I now feel prepared to write something for the "From the Zones" community project hosted at FATE SF.
First up, a few unusual and dangerous phenomena which may be encountered in a Zone. (Game mechanics are minimal, but are phrased in terminology compatible with any version of D&D or similar games.)
Not all regions of a Zone tolerate the usual spatial constraints of reality. Loose angles are one such spatial anomaly. They manifest as a sporadic, concertina-like shuddering, opening and closing motion in one of the angles of a seemingly solid object, for example the frame of a door or the corner of a building.
This unusual phenomenon has been reported by several stalkers who have witnessed it at a distance. No reports yet describe the effects (if any) of approaching a loose angle.
Small (1cm) geometric lumps found on otherwise flat surfaces, typically the interior walls of buildings. It is usual for clusters of several dozen nodules to be found in a small area. The nodules appear to be made out of the same material as the surface on which they appear -- apart from their incongruity with the rest of the surface, they have no distinguishing features.
Stalkers who come within 3m of an inversion nodule begin to feel an extremely unpleasant tugging sensation in their internal organs, as if they were wrapped with wires which are being pulled toward the nodule. At this point, a saving throw is required to avoid throwing up. Anyone coming closer than 2m experiences the full horror of the inversion nodules' effect: the simultaneous explosion and complete inversion of the body, over the course of a second or two. This process is, obviously, fatal. Victims are reduced to a sprawling mass of entrails -- the internal organs and bones on the outside, the skin and hair on the inside.
Patches of ground covered with gravel or rubble sometimes manifest the phenomenon known as scarlet scree -- streaks of bright red colouration which glow faintly in darkness. It is not known whether the scarlet colouration is caused by a covering of some sort (a fungus? dust? dye?), by an inexplicable transformation of matter, or by some kind of light trick.
Unlucky stalkers who come into contact with scarlet scree feel a fizzing sensation in the blood vessels close to their skin, and must make a saving throw or suffer several long-term, deleterious effects. If direct skin contact (with any part of the body) is made, the saving throw is at -2. Those who fail the save experience no immediate side-effects, beyond the blood-fizzing sensation, but suffer a loss of all body hair within 1d6 weeks, a gradual reddening of skin pigmentation (over the course of 1d6 months), and a loss of 1d4 points of CON one week after the contact.
Patches of scarlet scree have been reported to move around slowly. The actual movement has as yet not been directly witnessed as it occurs, but maps and the word of experienced stalkers show that it can disappear at one spot, and move to another nearby location.
It is rumoured that scarlet scree reacts with water, causing a violent explosion.