http://netflix.com/If you don't know what this is, it's basically what you've been missing your entire life.
$9 gets you unlimited mail rentals of _tons_ of movies + online watching of some. $13/$16 lets you rent 2/3 movies at a time.
The selection is fairly good, I think. And the online streaming works amazing (I only get 200kb/s most of the time, and I watched The Royal Tenenbaums at DVD quality with nearly no wait).
It has a two-week free trial (similar to WoW, you have to cancel it by the end or it will charge you).
I reiterate:
MOVIES FOR $9/MONTH