Constants in engine_stocks.inc

List of constants

If visible = 1, entity will be set to be visible, else invisible.
stock set_entity_visibility(entity, visible = 1) 
{
    entity_set_int(entity, EV_INT_effects, visible == 1 ? entity_get_int(entity, EV_INT_effects) & ~EF_NODRAW : entity_get_int(entity, EV_INT_effects) | EF_NODRAW);

    return 1;
}
Backwards compatible
Hurts/Kills players in a sphere, like an explosion, Multiplier determines damage.
stock RadiusDamage(const Float:fExplodeAt[3], iDamageMultiplier, iRadiusMultiplier) 
{
    return radius_damage(fExplodeAt, iDamageMultiplier, iRadiusMultiplier);
}
Gives you a velocity in the direction a player is looking, iVelocity is the multiplier.
stock VelocityByAim(iIndex,iVelocity, Float:vRetValue[3]) 
{
    return velocity_by_aim(iIndex,iVelocity,vRetValue);
}