Hulu in 2008. It used to be completely free, browser based, with no ads. It was the only way for me to watch new episodes of Heroes every week.
I first saw The Simpsons when it was a sketch on The Tracy Ullman Show, and thought it was the funniest thing I'd ever seen. I was ecstatic when I learned they were making a full cartoon of it. I watched it religiously up until about season fifteen or sixteen before I finally switched to every once in a while like we all eventually did.