(written from a Production point of view)
Hulu is an American online streaming service that offers a selection of TV shows, movies, and other media. Subscribers can access episodes in HD (when available) from a number of US networks including NBC, Fox, and ABC.
All of The Original Series, Star Trek: The Next Generation, Star Trek: Deep Space Nine, Star Trek: Voyager, and Star Trek: Enterprise are currently available via Hulu. The Original Series and the first two seasons of The Next Generation are available in remastered high definition.