// Variable Blur
// Jorge Fuente-Alba
// PS 2.0

//#CROSS-SAMPLING : reads pixels it may be writing.
//#BORDER-MODE : samples pixels outside bounding box
//#PARAM float blur 2.5 : Blur : amount of blur.

// Foreground texture
texture ForegroundTexture;

// Foreground sampler
sampler2D foreground = sampler_state {
    Texture = (ForegroundTexture);
    MinFilter = Linear;
    MagFilter = Linear;
    MipFilter = Linear;
};

// Parameter variables
float pixelWidth;
float pixelHeight;
float blur;

// Effect function
float4 EffectProcess( float2 Tex : TEXCOORD0 ) : COLOR0
{
    // Add the front and back pixels
    float4 here = tex2D(foreground, Tex.xy)*0.25;
    float4 left = tex2D(foreground, float2(Tex.x - (pixelWidth*blur), Tex.y))*0.1875;
    float4 right = tex2D(foreground, float2(Tex.x + (pixelWidth*blur), Tex.y))*0.1875;
    float4 top = tex2D(foreground, float2(Tex.x, Tex.y - (pixelHeight*blur)))*0.1875;
    float4 bottom = tex2D(foreground, float2(Tex.x, Tex.y + (pixelHeight*blur)))*0.1875;

    return here + left + right + top + bottom;
    //float4 src = tex2D(foreground, Tex.xy);
    
    //return result * (1-src.a) + src;

}

// ConstructEffect
technique ConstructEffect
{
    pass p0
    {
        VertexShader = null;
        PixelShader = compile ps_2_0 EffectProcess();
    }
}
[/code:dlvy8o5v]

Example .cap using it: [url]http://www.udec.cl/~jfuente_alba/blur_example.cap[/url]

// Dropshadow blurred
// David Clark (edited by Louis Ferina, Jorge Fuente-Alba)
// PS 2.0

//#CROSS-SAMPLING : reads pixels it may be writing.
//#BORDER-MODE : samples pixels outside bounding box
//#PARAM float offsetX 5.0 : X Offset: X offset of dropshadow.
//#PARAM float offsetY 5.0 : Y Offset: Y offset of dropshadow.
//#PARAM float blur 2.5 : Blur : Blur amount of dropshadow.
//#PARAM float opacity 0.5 : Opacity : Opacity of dropshadow.

// Foreground texture
texture ForegroundTexture;

// Foreground sampler
sampler2D foreground = sampler_state {
    Texture = (ForegroundTexture);
    MinFilter = Point;
    MagFilter = Point;
    MipFilter = Point;
};

// Parameter variables
float offsetX;
float offsetY;
float pixelWidth;
float pixelHeight;
float blur;
float opacity;

// Effect function
float4 EffectProcess( float2 Tex : TEXCOORD0 ) : COLOR0
{
    // Add the front and back pixels
    float2 Tex2 = Tex;
    Tex2.x -= offsetX * pixelWidth;
    Tex2.y -= offsetY * pixelHeight;
    float4 here = tex2D(foreground, Tex2.xy)*0.25;
    float4 left = tex2D(foreground, float2(Tex2.x - (pixelWidth*blur), Tex2.y))*0.1875;
    float4 right = tex2D(foreground, float2(Tex2.x + (pixelWidth*blur), Tex2.y))*0.1875;
    float4 top = tex2D(foreground, float2(Tex2.x, Tex2.y - (pixelHeight*blur)))*0.1875;
    float4 bottom = tex2D(foreground, float2(Tex2.x, Tex2.y + (pixelHeight*blur)))*0.1875;

    float4 result = (here + left + right + top + bottom )* opacity;
    float4 src = tex2D(foreground, Tex.xy);
    
    result.rgb = 0;
    
    return result * (1-src.a) + src;

}

// ConstructEffect
technique ConstructEffect
{
    pass p0
    {
        VertexShader = null;
        PixelShader = compile ps_2_0 EffectProcess();
    }
}
[/code:wt3j76il]

This wouldn't have been possible without the original shader OR the edit. Props to David and Louis.
Also, the opacities were somewhat arbitrarily chosen, they're not mathematically correct (but they look good enough for me). Positions too are axis-aligned, which is bad. I'll update all of that once I read up on blur kernels.
Also: 5 tap only so don't go over 10 on blur size or you'll see banding. At size 2 it looks beYaootiful!

Now, this knowledge enables me to do that variable kernel-size blur I've been wanting. Coming soon!
Oh, example cap for dropshadow blurred:
[url]http://www.udec.cl/~jfuente_alba/dropshadow_blurred.cap[/url]