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
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.
You can see the rest at the source
I haven't seen this anime yet, but I want to based on the Snuggle edit alone.