I'm just getting into this. One of my goals is to replace DirecTV with antenna and streaming. I decided to try out Amazon first as I order enough from them that the $79 fee for Prime ended up being a wash as Prime includes free 2-day shipping.
They don't have a whole bunch of free content, but I've been enjoying it so far. Everything I've watched below is part of the free shows.
I didn't start watching Mythbusters until a couple years ago, so I've been catching up with the older shows.
I never regularly watched Buffy the Vampire Slayer or Farscape when they aired due to scheduling conflicts(in those primitive pre-DVR days!) and have enjoyed watching them from the beginning.
Started watching Sons of Anarchy, quite good. Only the first 3 seasons are free (I'm only halfway thru season 1 so it'll be a while before that's an issue).
Just finished the first 2 seasons of Red Dwarf. They've got a bunch of other BBC shows as well. The new Doctor Who episodes look far better on Amazon than they did when I watched them on DirecTV (where BBC America is an over-compressed standard def channel).
All the Star Trek series are there. TOS is the remastered HD version, Enterprise is also in HD. The others are SD.
Like ST:TOS, the Twilight Zone episodes are the remastered HD versions and they look noticeably better than the DVDs I have.
There's a bunch of free movies as well though I've only watched one so far, Forbidden Planet - I was quite impressed at the special effects.
I'd calculated before that dropping DirecTV and getting content off the air or paying for it via streaming or blu-ray discs would save me $800-900 a year over what I'm paying now. The reason I haven't done it yet is my old HDTV. It only supports component video and I'd like to set the DVR software up on a Mac mini, which wants HDMI. My HDTV does have a VGA port, it maxes out 640x480 and the Mac won't go lower than 800x600. Between an unplanned for hospital visit and a new AC unit the new TV is on the back burner.