Since last summer I've been watching Star Trek: The Animated Series, at a leisurely pace. Now I'm almost through, with just a couple episodes left. It's a pretty fun show, though kinda campy in many ways.
The series, made in the early 70s, largely follows the original show's premise and style. All the main cast are included and voiced by the original actors. (The only notable exception being the absence of Chekov, who's been replaced by a silly looking alien with an even sillier voice.)
Not being restricted by the special effects constraints of the time, a lot of the stories and creatures are perhaps a little more imaginative than what was seen in TOS. Unfortunately, a lot of the ideas are also a little... far-fetched, scientifically implausible, or just plain dated. (The canon status of the show has been apparently somewhat debated, according to Wikipedia.) But they episodes are still entertaining enough.
The animation... is not the greatest I've seen. Even by 70s standards. There's a lot of clumsy looking movement, and 'cheap' still shots. But hey, it's the stories and dialogue that matter.
So yeah, I think TAS is obviously a must for any Trekkie, but possibly might require a certain... sense of humour and appreciation for camp value to really enjoy.
And I really do need to get some of the other Star Trek series and finally watch them in full one of these days. But money and time are limited... Although I've seen all the movies many times, my knowledge of the shows is, alas, far more spotty. (And it's a shame they haven't made any new Trek content since the early 2000s... Yes, I stand by that statement.)