Favorite ones? Chris Klein/Mamma Mia was pretty hysterical.

Also, does anyone know who leaks them? I'm guessing the actor or someone they are.

I really liked the 12 Years A Slave audition tape Michael Richards did. Totally should have gotten that role over Fassbender.

<iframe width="420" height="315" src="//www.youtube.com/embed/GZSnB7yGylc" frameborder="0" allowfullscreen></iframe>

assistant directors will leak audition type of videos.
assistant directors are also create separate facebook page for that movie not fans.

Gerard Butler's audition tape for Dracula 2000 is something else.

<iframe width="420" height="315" src="//www.youtube.com/embed/Zdy7jdNE3SQ" frameborder="0" allowfullscreen></iframe>

I'll answer the obvious question: he looks like that because he was shooting a movie in which he starred as Attila the Hun.

I saw one with this asian girl and a big burlap sack, acupuncture needles, piano wire, and something about "kiri"

I thought it was for the Calvin Candie role in Django Unchained.