(written from a Production point of view)
All Star Trek DVD and Blu-ray Disc releases are available for rental.
Star Trek is available for online streaming.
Streaming for US customers of Star Trek: The Original Series (remastered), Star Trek: The Next Generation, Star Trek: Voyager, and Star Trek: Enterprise began July 1st, 2011. By September 2011, Star Trek: The Animated Series was available as well, though TAS wasn't part of the original overall CBS deal. Streaming of Star Trek: Deep Space Nine began on October 1st 2011. TOS-R and ENT stream in HD. 
In Canada, Netflix has Star Trek: The Motion Picture through Star Trek: Insurrection, as well as Star Trek: The Next Generation, which was added in late 2012. Star Trek was available at one point, but has now been removed.