Sethbling makes some crazy things in Minecraft, but I never would've thought something like this was possible. With a mere thousands of command blocks and redstone, he made a nether portal that lets you see the other side.
Click the video for the full explanation, but the crazy thing about this build is that it actually copies the chunk of the nether that you would be seeing into the overworld. It's not very functional of course, but it's a cool kind of magic trick in Minecraft.
Magic Nether Portal [Sethbling]