Humor will not be tolerated!! -Portia

Just how powerful was Mad-Eye's eye? Where did he get it?

In book 4 it sees through Harry's invisibility cloak, which wasn't hugely significant then, but after what we learn about the cloak in book 7, that's a ridiculously powerful magical object that Mad-Eye had.

It can also see through the back of his head, or wood (according to the HP Wiki). Maybe it just had the blanket ability to see all invisible things, no matter how they were made so?

I'm going to go with the idea that he had infrared.