As I've stated before, I don't really understand how Hulu.com works, but because they don't have every movie in existence readily available, I assume they're posting films and TV in a completely legal way. After looking through the collection, I've found some real gems.
Movies:Hulu.com used to have BSG, The Office and Chuck entirely online, but they've been taking episodes off since the seasons ended. I don't understand why, and I don't know if the full seasons will return.
The Fifth Element
Lost in Translation*
Open Your Eyes* (the far superior Spanish Vanilla Sky original)
And, of course, Hercules in New York
30 Rock (Season 2, plus clips)
ALF (Alien Life Form, Seasons 1 & 2)
Arrest Development (all three seasons!)
Buffy the Vampire Slayer (Seasons 1 & 2 only)
Firefly (all 14 episodes)
It's Always Sunny in Philadelphia (all three seasons)
* You need an account to view "intended for mature audiences" materials.