Is there a shader to only add an outline?

Bumping an extremely old post here, but considering the fact that I came by it when trying to figure this out myself here's for the others that might end up here:

(from the bottom of: http://wiki.unity3d.com/index.php/Silhouette-Outlined_Diffuse)

Shader.zip

This will give you an outline only, based on an input color. All you gotta do is add it as a separate material to your target object.

Old post, but also highest ranked on google for this topic. I just spent several hours writing a geometry shader that's much improved compared to a toon or silhouette shader by taking every edge in the mesh and turning those edges into quads rendered facing the camera. This means that it works on cubes in a satisfying manner. Be aware that this is not a mobile-friendly shader, due to using the stencil buffer and rendering the object twice, and requires forward lighting. It also does not render the object itself, just the outline (due to my own needs) but should be easily modifiable to render the original mesh as well.

Note that because verts are supplied going clockwise around the surface normal, single sided quads will only render 3 edges (the other two are facing away from the camera). Similarly, at hard corners, the corners will be noticeably thinner than other areas, but this is only a problem at very high thickness values (greater than 10, which is pretty darn thick). This has to do with the quad's corners not being fused with the neighboring triangle's quads.

Both of these problems are technically solvable, but wasn't worth the effort for my use, due to only being apparent in edge-cases I wasn't going to be using. The former can be solved by rendering the front and back face of each quad (and correspondingly increasing maxvertexcount by double). The latter might be solved by using triangleadj in the geometry shader portion, instead of triangle, and calculating intersection points. See: https://msdn.microsoft.com/en-us/library/windows/desktop/bb509609%28v=vs.85%29.aspx

If there's an issue with it rendering strangely, swap float4 parallel = end.pos-start.pos; for float4 parallel = start.pos-end.pos; which I had to do when I merged this shader into another for use as a replacement shader that handles particular special rendering effects. Why? No idea.

 Shader "Stencil/Outline" 
 {
     Properties 
     {
         _Color("Color", Color) = (1,0,0,1)
         _Thickness("Thickness", float) = 4
     }
     SubShader 
     {
     
         Tags { "Queue"="Geometry" "IgnoreProjector"="True" "RenderType"="Transparent" }
         Blend SrcAlpha OneMinusSrcAlpha
         Cull Back
         ZTest always
         Pass
         {
             Stencil {
                 Ref 1
                 Comp always
                 Pass replace
             }
             CGPROGRAM
             #pragma vertex vert
             #pragma fragment frag
             #pragma multi_compile_fog
             
             #include "UnityCG.cginc"
             
             struct v2g 
             {
                 float4  pos : SV_POSITION;
                 float2  uv : TEXCOORD0;
                 float3 viewT : TANGENT;
                 float3 normals : NORMAL;
             };
             
             struct g2f 
             {
                 float4  pos : SV_POSITION;
                 float2  uv : TEXCOORD0;
                 float3  viewT : TANGENT;
                 float3  normals : NORMAL;
             };
 
             v2g vert(appdata_base v)
             {
                 v2g OUT;
                 OUT.pos = mul(UNITY_MATRIX_MVP, v.vertex);
                 OUT.uv = v.texcoord; 
                  OUT.normals = v.normal;
                 OUT.viewT = ObjSpaceViewDir(v.vertex);
                 
                 return OUT;
             }
             
             half4 frag(g2f IN) : COLOR
             {
                 //this renders nothing, if you want the base mesh and color
                 //fill this in with a standard fragment shader calculation
                 return 0;
             }
             ENDCG
         }
         Pass 
         {
             Stencil {
                 Ref 0
                 Comp equal
             }
             CGPROGRAM
             #include "UnityCG.cginc"
             #pragma target 4.0
             #pragma vertex vert
             #pragma geometry geom
             #pragma fragment frag
             
             
             half4 _Color;
             float _Thickness;
         
             struct v2g 
             {
                 float4 pos : SV_POSITION;
                 float2 uv : TEXCOORD0;
                 float3 viewT : TANGENT;
                 float3 normals : NORMAL;
             };
             
             struct g2f 
             {
                 float4 pos : SV_POSITION;
                 float2 uv : TEXCOORD0;
                 float3 viewT : TANGENT;
                 float3 normals : NORMAL;
             };
 
             v2g vert(appdata_base v)
             {
                 v2g OUT;
                 OUT.pos = mul(UNITY_MATRIX_MVP, v.vertex);
                 
                 OUT.uv = v.texcoord;
                  OUT.normals = v.normal;
                 OUT.viewT = ObjSpaceViewDir(v.vertex);
                 
                 return OUT;
             }
             
             void geom2(v2g start, v2g end, inout TriangleStream<g2f> triStream)
             {
                 float thisWidth = _Thickness/100;
                 float4 parallel = end.pos-start.pos;
                 normalize(parallel);
                 parallel *= thisWidth;
                 
                 float4 perpendicular = float4(parallel.y,-parallel.x, 0, 0);
                 perpendicular = normalize(perpendicular) * thisWidth;
                 float4 v1 = start.pos-parallel;
                 float4 v2 = end.pos+parallel;
                 g2f OUT;
                 OUT.pos = v1-perpendicular;
                 OUT.uv = start.uv;
                 OUT.viewT = start.viewT;
                 OUT.normals = start.normals;
                 triStream.Append(OUT);
                 
                 OUT.pos = v1+perpendicular;
                 triStream.Append(OUT);
                 
                 OUT.pos = v2-perpendicular;
                 OUT.uv = end.uv;
                 OUT.viewT = end.viewT;
                 OUT.normals = end.normals;
                 triStream.Append(OUT);
                 
                 OUT.pos = v2+perpendicular;
                 OUT.uv = end.uv;
                 OUT.viewT = end.viewT;
                 OUT.normals = end.normals;
                 triStream.Append(OUT);
             }
             
             [maxvertexcount(12)]
             void geom(triangle v2g IN[3], inout TriangleStream<g2f> triStream)
             {
                 geom2(IN[0],IN[1],triStream);
                 geom2(IN[1],IN[2],triStream);
                 geom2(IN[2],IN[0],triStream);
             }
             
             half4 frag(g2f IN) : COLOR
             {
                 _Color.a = 1;
                 return _Color;
             }
             
             ENDCG
 
         }
     }
     FallBack "Diffuse"
 }
 

Find and download the Strumpy Shader Editor in the Asset Store (it's free!) and you can probably get exactly what you want to.

http://wiki.unity3d.com/index.php/Shader_Archive

Try looking through these.

I recommend this post by Boolean:

http://forum.unity3d.com/threads/trying-to-make-a-silhouette-outline-shader.96005/#post-1408098

In my opinion, it's better than the Silhouette Ouline Diffuse shader from the wiki, it hasn't got the same overlap issues and the effect is also very cool.

Also take a look at my own modifications to this shader as detailed here:

http://answers.unity3d.com/questions/667483/silhouette-outline-shader-problem-with-overlapping.html