If I understand correctly, render textures are a feature of Unity Pro, and that is the best way to create mirrors, TV screens, and so forth. If I am wrong, please tell me. What I would like to know is if there is another, probably less-convenient, way to create a mirror with the free version of Unity. If I need to write a custom script, I would appreciate a basic outline of how to write it. Even better, if you know of a free script package that does this, please tell me.
You are correct in that render textures are the best way to do mirrors in Unity. Coincidentally, a game that I worked on recently required the use of mirrors (though our mirrors would also reflect things that may not exist in the world). We were also developing with Unity Free, so we had the same problem as you.
Our solution was as follows (recommended to us by a professional who works with Unity): write a script that copies a GameObject, and strips all Monobehaviour, Physics, etc. components off that object (as well as it's children). Basically all that should be left is the transform and the mesh/material data. Then, depending on the mirror location and orientation, you can change the transform of this shell object to the appropriate location on the other side of the mirror. We used a nifty parenting trick that I can't remember the exact code for.
For any dynamic objects, you'll have to write a script to update the 'mirror object' based upon the original object's location and animation. You could either store a reference to the original object in your mirror object, or vice versa. This is probably the hardest part of the process.
As for the mirror itself, you can apply any kind of transparent material to it in order to make it look a bit more like the surface you want.
The whole process itself sounds daunting but when it comes down to it, mirroring in this way isn't actually that bad. It's when you want different things happening in the 'mirror world' that it gets messy.
Hope this helps :)
Maybe an old topic, but for those unity devs out there... they are bringing the stencil buffer to free version in untiy 4.6, this way, you can make basic mirrors with the ability to cull objects out of the other side.
AFAIK You Cant make a perfect one, but if you make a cubemap of the room and set the shader of the mirror to Reflective/diffuse, then apply the room cubemap, you get a good feeling of a mirror. you just gotta tweak the cubemap to get it perfect and make the face sizes more than 64 or it will look blurry.