Omigod. You're going to have to excuse me while I fangirl here. How did I not know Jensen could sing? Why is that man so perfect?! It's not fair. I completely agree with you that he should clone himself, because I need for some of that beauty for myself.
It comes from a gag reel or special features or something, (I can't actually see the video due to my internet) but if it's him singing Eye of the Tiger, then it's from a special features on a Supernatural dvd. :)