WireFrame Render

Modifier

With the newer versions of Blender (2.70 and up), the wireframe can be applied via a modifier.  Since it's a modifier, it applies for both renderers.

In the modifier menu, it is under the Generate column.

This produces a wireframe of the mesh you have selected.

The attributes of the wireframe can be modified in the modifier panel.  The most common would be also turning on the original mesh, which can be done by unchecking the Replace Original field.

An example final render of the wireframe (material offset 1) and the original mesh.

Cycles Renderer

This entry will detail the steps to render a clay and/or wire render of your mesh.  The first is the Cycles approach which makes use of an add-on which can be found here: Blender Artist Forum

For this example, we'll use suzanne as our complex mesh.

After adding the add-on, when you select a mesh, you'll notice in the left-pane toolbar that these options are available.  Selected Meshes will apply the clay material to selected mesh, and Create Wire Object will create a separate object that is the wire frame of the mesh.  Click Apply Materials for this to take effect.

Note once again, a new clay material is added to the Monkey, and a new wire_object is created as a separate curve.

Render of wire curve and clay material.

Render of only wire curve, but with an emissive material.

Blender Internal

Same concept but for the Blender internal renderer.

To render the mesh wireframe, simply create a new Material and press the Wire button shown below.  To get even fancier, you can create two materials, and assign both to the entire mesh to create a solid with a wireframe (last image).

Solid with wireframe render.