Batch Renderers

Idea: Create command-line rendering software that reads the description of a 3D scene from files, renders a corresponding image, and writes this to disk.
Pros

Can be used with many "front ends" such as 3D editors and visualization software
Doesn't need a graphical user interface
Obvious choice for use in rendering farms

Interactive control of the rendering process is hard
Such a standard rendering software may not support some required special features

PhotoRealistic RenderMan by Pixar (language: RenderMan Interface Bytestream Protocol RIB, special programming language for shaders); there are several RenderMan compatible renderers; plugins allow to write RIB files with virtually every major 3D modeling and animation software
mental ray by mental images (.mi files), stand alone or part of Maya, XSI, and 3ds max
POV-Ray, freeware for a broad range of operating systems
Maxwell Render Engine for physically correct materials, caustics, and dispersion; bundled with graphical front end
and many more

Alternative: Use a programming interface (API) to invoke and control the rendering software from another program

also included in PhotoRealistic RenderMan and mental ray (library version)
finalRender by Cebas, which is available as a plugin für 3ds max and forms the basis of the standard renderer of Cinema 4D
and many more

Post-Processing

Idea: Render as usual, but store additional data per pixel, for instance depth, glow intensity, object number. (The newest graphics cards offer this functionality for real-time applications, too: Multiple Render Target, MRT)
Examples

Combine computer graphics with real film: people in a virtual spaceship, virtual flying saucers over New York

Write as single files: every specular contribution of every light, every shadow contribution of every light, and so on
Even simpler, render into a multi-layer Photoshop picture file or AfterEffects movie file
Pro: Lights, shadows, colors can be edited quickly in image manipulation software, with no need to re-render