(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 1 July 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 1 October 2011. TOS-R and ENT stream in HD. 
In Canada, Netflix has all of the Next Generation movies for streaming. Netflix also had Star Trek: The Next Generation, which was added in late 2012, then removed on 1 December 2013. It was re-added to Netflix Canada in July 2014, later to be taken off in early 2016. Movies I-VI, Star Trek and Star Trek Into Darkness were available at one point, but are no longer available.