Some of these mix the opening song (on top) with other anime intros or with other various videos. Some are just extremely funny.
Have a look
Attack on Miffy. Bunny rabbit and her firends become hardcore.
Spilled my coffee during this one, Attack on Snuggle.
Hell, it even makes the opening of JoJo's Bizarre Adventure more powerful.
King of the Hill is now Attack on the Hill.
Madoka opening, perfectly edited.
Pretty Cure is badass for the very first time.
And finally the weirdest edit doesn't even have anime, it's.. different.
Shingeki no Kyojin 進撃の巨人 - OPENING HD [YouTube]