For the uninitied, click on the link for the meaning of M.A.D
To start off this thread, I shall repost the legendary Nursery Rhyme OP which spawn off numerous parodied MAD.
1) Mahou Shoujo Lyrical Nanoha
2) Lucky Star Rhyme
3) Kagami Tsunde Rhyme
4) Konata Rhyme
5) Haruhi Melancholy Rhyme
6) Rosen Maiden Rhyme
I believe there are much more out there other than the 6 i posted. ^_^
And lastly, let's not forget this award-winning MAD based on Princess Tutu:
Hold Me Now