http://www.southparkstudios.com/
Southpark guys have made a new site and have the episodes that have been aired legally free to watch on their site
Currently not available in some countries yet but they are working on it, here in NZ it is coming soon, not sure if it will work in the UK yet but will be cool if it did

Coming soon to the UK it would appear.

Probably just US only till they sort all the legal rights etc down properly, Cool though. Hopefully other companies will get the idea that once you done the tv and then the dvd in terms of a series you have bled it dry already so why not, it gets more people into your show for when you do the next series.