Reflection probes(in both engines) let you have nice reflections at a somewhat cheap rendering cost, and work great for things like have the cars outside reflect/mirror the street lamps, but when in tunnels, reflect the tunnel lighting(which may be on the sides instead of the top). I mean the actual geometry though, not just the lighting/shading part.
Your mention of reflections reminded me of screen space reflections, where the game flips and distorts parts of the scene that has already rendered and paints it onto reflective surfaces to provide the appearance of a realtime reflection that updates as objects and the environment change. The major downside to this technique, however, is that because it's re-using already drawn pixels it cannot reflect anything that isn't on the screen. So you can usually end up recognizing this technique in games where a reflection seems to "stop" right in the middle of the shiny surface and change into a more generic color or cubemap.
The major upside, of course, is it is a far cheaper effect then actually rendering the scene from the perspective of each reflecting object to generate true reflections.
Yup, I remember doing things like this. You usually needed the stencil buffer too in order to make the reflections only draw on the reflection object instead of all over.
I remember using a quick matrix transformation plus the stencil buffer to get quick projection planar shadows. You basically draw the object a second time, but using a specially made transform matrix. This matrix would flatten the object onto a plane in space, from the point of projection from some light(position). Then, as long as you used the stencil buffer, the shadow would only be on that plane. The catch is that this trick only works well when your ground is actually a plane, as it quickly get expensive to draw the object on more than a couple of pieces of geometry to get shadows, which ends up leading to other shadow techniques.