Hiding part of mesh or texture

What is the best way to make a mesh or texture partially invisible?
I want to make a sprite seem to be able to move smoothly between planes which are at different angles, and I guess the best method is making the part of the quad that is not touching the plane invisible, and adding a copy of it to the other plane.

Modify the alpha colour on the material associated with it - presuming you are using a shader (like a transparent diffuse) that allows this.

Example:

 renderer.material.color = new Color(render.material.color.r, 
     renderer.material.color.g, renderer.material.color.b, alpha);

Where alpha is between 0 and 1

Do what mike says or you can use transparent shaders if you want good looks like in maya.

Hard surface shader this is a really good free source to go with.

You can use this: unifycommunity.com