Item description:

X Ray mod allows you to see through ‘walls’ in Minecraft by removing blocks from your field of vision that don’t interest you. Want to see through the ground and note where the diamond is? Use your powers of X Ray vision to hunt for diamond deposits, find caves and generally hack the heck out of your Minecraft experience.

1.Open your minecraft.jar with winrar
2.Make sure meta-inf is deleted
3.Open the download zip file(The xray mod folder)
4.Drag all the contents into the minecraft.jar folder
5.Then open the modloader folder you downloaded.
6.Do the same drag all the class's into the minecraft.jar
And your done

Press X to toggle.
You can customize 'terrain xray.png' however you like.
Doesn't work in multiplayer.
Doesn't see through custom blocks.
Untested with custom or HD textures.