Here's an anaglyph compositor I've made some time ago but forgot to post here (sorry).

How does it work:
Two cameras, slightly apart and linked to a node, view the scene from two slightly different angles. The tricky part was to overlay the left view (red) and the right view (cyan).
- The "right" camera is used in a "CreateRenderTexture";
- This texture is named, so it can be passed in an Ogre script;
- The "left" camera uses a compositor, which render the scene and call a material to display it on screen;
- This material has two texture units: the one created by the compositor, and the named render-texture;
- These two texture are then passed to a shader, which converts them to greyscale and mix the red chanel from the left image and then green/blue channels from the right image;
- And the compositor displays the result fullscreen.
Maybe not the best method, but it works

Here a zip with the source and the needed resources (textures, script, etc..)
http://keleb.free.fr/codecorner/downloa ... hCompo.zip
Have fun!