Cart CGD

here is the quick test/example I made

http://psycho-designs.com/cgd/normalTest.zip

[13MB-ish]
it includes a low res .obj, a normal map .tif and an occlusion map .tif [that could be applied as a diffuse map for now to enhance the details]

Here is a preview of the Normal map effect

There are so many different ways to create normal maps, each with their pros and cons.. most of which involve modeling a high resolution model to translate the details of which into the normal map that will make the low res look like the high [example above]
here are the techniques I used in the past and what I think about them:

-ZBrush:
Pros:

• can handle millions of polygons which allows for extreme details [wrinkles!]
• you can go up and down in subdivision levels so you can work at any resolution, then go down to the base mesh, export it and generate the normal map for it
• gives so many options for the creation of normal maps [like the pre-sharpening details and precise RGB control]
• can generate cavity maps
• you can use bump maps to sculpt the details that will later be included in the normal map

Cons:

• not ideal for hard surface models [weapons, cars]
• its very tricky to generate normal maps out of multiple high res meshes for one low res mesh [like the example above, the little straps are gray because they are separate meshes [subtools] they couldn’t have been included in the normal map if it was generated within ZBrush, which is needed to have a truely low res model] <-[I'm not sure if that makes sense, let me know if it doesn't]
• can’t change the vertex count [extruding faces, splitting edges, deleting vertexes] of the the model

-Maya [and presumably Max]:
Pros:

• You can have all modeling tools Maya offers to create your high res model. Which means its good for hard surface models
• Can generate other maps like occlusion and displacement maps [displacement maps don't work in games but they are useful to enhance the details of the textures]

Cons:

• Can’t handle high res meshes from ZBrush [1 million and above] no matter how much ram you throw at it [learned that the hard way]
• Clunky controls/interface for generating [baking] the maps.
• Can be unstable at times [Save!]

-XNormal: [me preferred choice! and what was used for the example above]
Its a free program for windows that can be downloaded here: http://www.xnormal.net/Downloads.aspx
Pros:

• Generates beautiful Normal maps, better looking than the techniques above.
• Its Free! and easy to use [video tutorials are available on the site]
• Can handle the heavy meshes of ZBrush.
• You can import multiple high res meshes to generate a normal map for one or more low res meshes. which means you can have the organic high res character mesh from ZBrush and the hard surface armor and straps from Maya and generate one normal map that includes all of the meshes.
• Generates countless maps that include: occlusion, cavity, thickness, hight, complexity, and convexity map [no idea WTF that is]

Cons:

• its for windows only as far as I know
• nothing yet [this is only my second time using it, I'll be updating this list once I find other cons]

Nvidia’s Photoshop plugin:
this is whats mentioned in the link posted by Ramy. its a plugin that generates normal maps out of gray scale bump maps.
Pros:

• Useful for details that are easier to paint as texture than model [like hair and text]
• its free!
• can be easy to use if there is not much tweaking needed

Cons:

• hard to control or predict the results [complicated interface]
• painting details in 2D is very limited compared to actually modeling/sculpting those details in 3D