there are many. here are a few programs i find above the rest in one way or another:

kink has some of the best bdsm on the web.

xxxrewards has a couple of the oldest and largest sites around. you might know them from their twink site, but their str8 sites are good, too.

twistyscash the sites offer gorgeous models and exclusive content.

perfectgonzo has high quality exclusive HD content in all their sites.

naughtybank has 2 good hot wife sites, although naughtyallie is the bigger of the 2.

adultplatform is the program for videobox, one of the biggest DVD archives on the web, and also one of the cheapest.

bangbros has a bunch of exclusive reality sites - in fact, they were pioneers in the reality thing.