Years before Inception (movie)

Here's a short list of TV episodes where similar concepts of dream manipulation and recursive realities were explored decades ago:

The Avengers 1967:
Deaths Door (A series of Diplomats are drugged and forced to participate in their own nightmares. On waking up, events unfold just like the nightmare - all the way up to the Diplomatic event)

Star Trek: The Next Generation: Season 6:
Ship in a Bottle (A sentient hologram of Dr. Moriarty tricks the crew into thinking they are back on the ship after visiting the Holodeck. In actuality they are in a hologram ship inside another hologram.)

Frame of Mind (An officer is trapped and taken prisoner while on a mission. He is then drugged and his dreams are tampered with. He starts thinking that his actual life on the Enterprise was a delusion and is convinced to find closure with his delusional characters by killing them in his mind among other things)

Star Trek: Voyager: Season 2
Projections (The ships holographic doctor is convinced into believing that he is not a hologram but the actual hologram designer who has got lost in his own mental simulations on a Star base near Jupiter. He is asked to give up control of the ship to end the simulation)

Similar types of episodes in Mission Impossible Season 1 (1966) and 2.

[Update 1: Also see DreamWithinADream]