At the end of S03E21 "Hollow Pursuits", Reginald Barclay deletes all his holodeck programs, except for program nine.
What was in it?
We don't know and are never told. I always felt that the strong implication is that it was the program with Troi (with whom Barclay is obsessed).
The original show-script indicates that he switched from the (unnamed) program with just Troi to "Program 15" which contained both Troi and a more complex setting with more of the crew. It's certainly not unreasonable to assume the less complex program was designed earlier and thus had a lower number, possibly number nine.
BARCLAY: Mmm, nice... but I'm in the mood for someplace a little more...unusual... (beat) Computer. Run Barclay Program Fifteen.
We stay on Barclay, but lose Holo-Troi, as the surroundings change to
9 EXT. GLADE (HOLODECK) A glade with a few small hills, rock formations, some trees, grass, a pond. As different as can be from the world Barclay lives in... peaceful, green, remote... a warm breeze flutters through the trees... Barclay takes a deep, cleansing breath... turns to see ---
Barclay's Program Nine makes an appearance in the eponymously named story in "Strange New World VII", written by Russ Crossley: A trader has acquired the program and attempts to sell it to Quark. It appears to be the program we saw in the TV show.
The raven haired human female stood before him with her arms thrown wide, her dusky curls trailing about her milk-white shoulders like a cascade of fine black silk, and her dark uncomprehending eyes gazing into the distant forest surrounding the glade. The artificial breeze that blew across them caused her flowing ground-length robe to billow about her as if she floated on a thunderhead. The only thing she’d said since appearing was, “I am the Goddess of Empathy.”
There is no canonical information as to what "Program 9" contains. There was an award-winning short story by Russ Crossley called "Barclay Program 9" submitted as part of the Strange New Worlds competition, but it's just a bit of fan fiction (technically extended universe, as the contest was licensed).
The question of what "Program 9" contains is not a new one — it's something that Trek fans have debated since "Hollow Pursuits" first aired.
Two fairly popular theories, which have appeared in forums over the years:
To support theory #2, some have pointed out that Barclay is not a particularly impressive crew member and, while technically adept, would more likely be posted to a less stellar starship than the flagship.