Drop any of these standard shader derived materials into your project to be able to see them through walls. Lots of sliders and stuff for customization.
Also includes a special standard derived material for blocking see through objects. This is so you can have a terrain or something that doesn't show the player through the floor, but you can have walls and such that do.
Does not require a second camera. Does not require the model to be duplicated. Very fast and efficient shader / material setup that doesn't break all your stuff.
Note: In deferred rendering, the xray-blockers cannot be used.