I made a quick rain effect. I decided to post the code here.

Code:

---

var droplets;
string raindropBMP = <raindrop.bmp>;

action rain
{
	my.passable = on;
	my.transparent = on;
	my.alpha = 25;
	while(my.z > 0)
	{
		my.z -= 20 * time;
		wait(1);
	}
	droplets -= 1;
	ent_remove(me);
}

function raindrop()
{
	var rainVec[3];
	while(!player) { wait(1); }
	while((player) && (droplets < 400))
	{
		randomize();
		rainVec.x = player.x + random(800) - 400;
		rainVec.y = player.y + random(800) - 400;
		rainVec.z = player.z + 200;
		ent_create(raindropBMP,rainVec,rain);
		droplets += 1;
		wait(1);
	}
}  

---

All you need to do is put this in the script somewhere. The player entity must be followed by the camera in order for this to work properly. I think the nexus might have to be increased (or decrease droplets). And the player pointer has to be replaced by what ever pointer is used by your player.
raindrop(); is called in main()

Enjoy. I'll want credit, but its free for you, commercial or not

Oh, it looks better in run time than in the screenshot

there is not much of the actual effect in the screenshot, but its nice. U should still update the screenshot ^^

XTREME

Thank you, Alco!
I'm using similar code for rain, this is cut from it:
(you have to edit this before using, I cut it unchanged from my proj.)
Code:

---

...
bmap rain_mist_pic = <smoke.tga>;
function rain_mist_()
{
	my.alpha -= 5 * time;
	if(my.alpha < 0) 
	{
		my.lifespan = 0;
		return;
	}
	my.size += 2*time;//*0.5
}
function rain_mist()
{
	temp.x = random(2) - 1;
	temp.y = random(2) - 1;
	temp.z = random(0.5) + 0.5; // play with this value
	vec_set(my.vel_x,temp);
	my.bmap = rain_mist_pic;
	my.size = 5+random(5);//5
	my.flare = on;
	my.bright = on;
	my.move = on;
	my.gravity = 0;
	my.alpha = 70;//70
	my.function = rain_mist_;
}

bmap raindrop = <drop5.tga>; 

//RAIN effect function
Function eff_rain_()
{
	if(content(my.x) == content_solid) // the particle is inside a solid?
	{
		effect(rain_mist, 1, my.x, nullvector); // generate mist
		 // generate mist
		my.lifespan = 0; // don't allow the particles to continue their movement if they have hit something solid
		return;
	}
  my.skill_b += 0.06;//0.03
  my.skill_a = 1  - my.skill_b;//6
  if(my.skill_a < 0) {
    my.lifespan = 0;
  	}
}

Function eff_rain()
{
    if(my.skill_a < 0) { my.skill_a = 0; my.skill_b = 0; }
    my.bmap = raindrop;
    my.vel_x = random( 10 )-5;
    my.vel_y = random( 10 )-5;
    my.vel_z = -40 ;
    my.size =14 ;
    my.move =1 ;
    my.alpha = 100 ;
    my.x += random(CELL_SIZE * 16)-CELL_SIZE * 8;
    my.y += random(CELL_SIZE * 16)-CELL_SIZE * 8;
    my.z += random(CELL_SIZE * 2)-CELL_SIZE;
    my.transparent = on ;
    my.gravity =0 ;
    my.bright =on;
    my.function = eff_rain_;
    my.flare =1 ;
}
...
...
	var myPos[3];
	while(1) {
		while(RAIN_NEEDED==on)&&(MAP_VIEW_ACTIVE==off) {
			vec_set(myPOS,camera.x);
			temp.x = COS(camera.pan);
			temp.y = SIN(camera.pan);
			temp.Z = 600*COS(camera.tilt);
			MyPos.X = myPos.X + temp.Z*temp.X;
			MyPos.Y = myPos.Y + temp.Z*temp.Y;;
//			MyPos.Z = myPos.Z + 600*SIN(camera.TILT);
			MyPos.Z = Camera.Z + 100;//800;200
			effect(eff_rain,50 ,MyPOS,nullvector);
			if (moon.alpha > 50) { moon.alpha-=time; }
			moon.alpha = max(moon.alpha,0); 
			if (moon_flare.alpha > 50) { moon_flare.alpha-=time; }
			moon_flare.alpha = max(moon_flare.alpha,0); 
			wait(1);
		}
...

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