Hulu

Hulu is a great way to watch full episodes of the Simpsons among many other programing. There are about only 3 commercials per 20 minutes of viewing. It has a good amount of episodes and if you miss a new episode Hulu has it up the next day. The video quality is amazing and you can watch anything in Standard Definition (360p) or High Drfinition (480p). Also, Hulu provides closed captioning for all of it's media which is most likely a first in streaming video.

Here is the link to the Simpsons section of the site.