I don't think transsexuals are very "extreme" though I might have a hard time in front of a jury with Aunt Edna from Chattanooga.

In addition to the list that Max came up with (minus tranny porn) I'd vote (mostly from what we've been advised to avoid)

- incest
- scat
- sex acts where one party is in bondage (can't show consent)
- torture

Probably a ton of others as well, but the above are the ones that come to mind. Basically, just think of the third prong of the Miller test... if a reasonable sampling of "average" people in the community would find it patently offensive and without any redeeming social value, then it's obscene.