Local reflection in the screenspace. This is a minimal example, it can be extended, for example to make a mask of the intensity of reflections, blur, antialising, etc.
Pros: does not require the creation of additional cameras. You can use g-buffer, already created for other purposes (such as deferred lighting). Performance is almost independence of the scene complexity. Works with autoshaders.
Cons: Multiple artifacts – you need to mask them somehow (possible blur). Reflect only what is seen on the screen. “Voracious” raytrace algorythm.
I have included two version of shader in the source code. They are a bit different in the raytrace cycle. The difference will be noticable while distancing the camera.