Parallax mapping
Warning:Parallax maps are disabled in every Valve game, although since 
an (unfinished) shader exists named 
ParallaxTest, which can be used as an alternative.
Note:This downloadable shader only works in the 
Source 2006 engine.Note:Thexa4's PBR shader works in 
Source 2013.Parallax mapping (also known as offset mapping or virtual displacement mapping) is a shading technique that displaces the individual pixel height of a surface, so that when you look at it at an angle, the high points will obscure the low points behind them, making it look three-dimensional. The height data for each pixel comes from a 
$parallaxmap texture, which needs to be created for each parallax mapped material.
Parallax mapping is an enhancement of the bump mapping technique. It is only worthwhile for textures that have a depth of at least a few centimeters, like deep-set bricks or stone.
Creating a parallax mapped material
Creating a heightmap
Regardless of the way you are going to render them, creating parallax mapped materials always starts with creating a heightmap.
Adding a parallax map with the built-in support for parallax maps
Add these two lines to your Valve Material (.vmt) file:
Fill in the path and name of your heightmap on the first line, and a number between 0 and 1 on the second line. This number affects how deep the texture will look. 0.05 is an appropriate value for bricks.
Example VMT:
Adding a parallax map with the Parallax Occlusion Shader mod
Note:This mod is obsolete, and does not work properly with HDR. There is a reincarnation of the Parallax Occlusion Shader mod by the same author but that only works with models. A brush version is in the works. In the meantime, Parallax Occlusion Shader still works with HDR off.First, in the Valve Material (.vmt) file, after the line
"LightmappedGeneric"
add this line:
//"MN_PrlxLightmappedGeneric"
When playing a map with your parallax mapped material, comment out the first line. When compiling a map with your parallax mapped material in Hammer, comment out the last line. This (according to the author of the mod) is a workaround to a glitch in Source SDK.
Finally, add this line to your VMT file:
"$heightmap" "texture name"
Other parameters you may need:
"$heightscale", defines the "depth" of the effect, takes values 0.0—1.0, default=0.4
"$heightmapsample", default=16
Example VMT:
Adding parallax map with Thexa4's PBR shader mod
Note: Follow the tutorial Adding PBR to Your Mod to implement the shader into your source sdk project! This shader is compatible with HDR and with Source 2013.Warning: This shader drastically modifies how your materials look compared to lightmappedgeneric!!
To make the effect work you will need to have these 3 parameters in your VMT:
$parallax bool - Enables/Disables parallax mapping.
$parallaxdepth float - Controls the intensity of the parallax effect.
$parallaxcenterfloat - Center depth of the Parallax Map.
To make the parallax mapping work you will need to put your Heightmap into the Bump_map's alpha channel!!
Example VMT: