Netflix: Rent as many movies as you want for only $8.99 a month! Free Trial
When you sign up, they send you a disk for your PS3. I gather that they are working on a software package that you can download to your PS3 so that you won't need the disk, but for now you have to use the disk. Takes about 2 days or so to get the disk.
They've got a number different packages: the lowest package is $8.99, and lets you check out one DVD at a time, plus unlimited streaming movies. I think they have a $5.99 package that lets you get like 3 DVDs/month and you can stream 2 hours of video, also, but the $8.99 package seems like the best deal.
They don't have their entire catalogue available via streaming, but they've got like 17,000+ movies and TV shows available.