As of June 2015, episodes of the older Scooby-Doo cartoons are available on Amazon Instant Video. The newer series, "Scooby-Doo, Mystery Incorporated," is also available on both Amazon Instant Video and Netflix.
Amazon Instant Video has seasons one and two of "Scooby-Doo, Where Are You?" and all five seasons of "Scooby-Doo and Scrappy-Doo." They also have every season of "Scooby-Doo, Mystery Incorporated," which debuted in 2010.
Seasons one and two of "Mystery Incorporated" are available to stream on Netflix. Netflix also has many of the Scooby-Doo animated films.
Cartoon Network is planning to release a new series called "Be Cool, Scooby-Doo" in 2015. The network generally makes episodes of its cartoons freely available online the day after they air.