Unity3D compatible CG shader to use with an orthographic projector to create a square grid.

Grid.shader

Shader "Projector/Grid" {

	Properties {
		_Color ("Color", Color) = (1,1,1,0)
		_ShadowTex ("Cookie", 2D) = "black"
		_Size ("Grid Size", Float) = 1
	}
	
	Subshader {
		Tags { "RenderType"="Transparent" "Queue"="Transparent+100" }
		Pass {
			ZWrite Off
			Offset -1, -1
			Fog { Mode Off }
			ColorMask RGB
			Blend SrcAlpha OneMinusSrcAlpha
			
			CGPROGRAM
			#pragma vertex vert
			#pragma fragment frag
			#pragma fragmentoption ARB_fog_exp2
			#pragma fragmentoption ARB_precision_hint_fastest
			#include "UnityCG.cginc"
			 
			struct v2f {
				float4 pos : SV_POSITION;
				float2 uv : TEXCOORD0;
			};
			 
			uniform sampler2D _ShadowTex;
			float4 _ShadowTex_ST;
			float4 _Color;
			float4x4 _Projector;
			fixed _Size;
			 
			v2f vert (appdata_tan v) {
				v2f o;
				o.pos = mul (UNITY_MATRIX_MVP, v.vertex);
				o.uv = TRANSFORM_TEX (mul (_Projector, v.vertex).xy, _ShadowTex);
				return o;
			}
			 
			fixed4 frag (v2f i) : COLOR {
				return tex2D (_ShadowTex,  fmod (float2(saturate(i.uv.x),saturate(i.uv.y)), 1 / _Size) * _Size) * _Color;
			}
			
			ENDCG
		}
	}
}