Most of the discussion in this thread seems to be about D&D modules.
When I play FRPGs of a D&D-ish nature (eg 4e D&D, Torchbearer), I will use modules for maps, NPCs and situations. Here are three examples from 4e D&D play:
After all the sandbox/railroading threads over the past several weeks, in my most recent session I decided to try running an exploration-heavy scenario to see how it played. The starting point In the previous session of my 4e game, the PCs had arrived at a crude wooden house built on ruins...
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In our session yesterday the PCs returned to the Underdark. They are trying to reach the Soul Abattoir, with the goal of undoing Torog's hold over the souls of those who die in the Underdark. They are following a map that takes the form of a magical tapestry created in an ancient minotaur...
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The PCs in my 4e game have gone to the Feywild looking for the Glacial Rift of the Frost Giant Jarl, so they can destroy the Frost Giants who are massing, in alliance with Lolth and the Prince of Frost, to start a War of Seasons that will overthrow the Summer Fey and steal control over winter...
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Here is an example from Torchbearer 2e - the module is the T1 Moathouse, as adapted
here:
We played another session yesterday. We started with Golin and Fea-bella lost in the Troll Fens. Fea-bella's player recounted a prologue, and removed Hungry and Thirsty. I then told them that in escaping from the tricksy Troll Haunt they had lost track of time and distance, but that now, as the...
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I also use modules/scenarios in non-D&D-ish play.
I have used some Classic Traveller scenarios, but as presented they were pretty boring so I spiced up the situation: I added Aliens (TM) to Annic Nova; and I buried the complex in Shadows beneath 4 km of ice and made it the site of a contest over knowledge of ancient alien technologies and psionics, with the complex itself more of a backdrop:
Context Our group's most recent RPG sessions have been Prince Valiant, but yesterday we returned to our Classic Traveller campaign for the first time in (I would guess) 6+ months. I suggested Traveller for two reasons: (1) One of our group has been absent for a long time due to home renovation...
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Over the past month or so my group has been able to play two sessions of Classic Traveller. The PCs had travelled to the icy world of Zinion looking for the ruins or relics of an ancient alien civilisation. They knew that the aliens had lived on the world around 2 billion years ago, and had...
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In both cases, I more-or-less shared the map with the players, as
exploration was not really what play was about.
Generally, the closer a published module/scenario is to a
situation than to a
plot, the more useful I find it. This is how I used various scenes from the MHRP Civil Wars book:
GMed first MHRP session on Sunday
The consistently best situations I know are for Prince Valiant, and I've used lots of them. One of the most complex and most interesting is The Crimson Bull:
Prince Valiant actual play
For play where the focus is more on character than on situation - eg Burning Wheel - pre-authored scenarios/situations are less use, although I did have some success adapting the d20 scenario Maiden Voyage to BW:
Maiden Voyage (Penumbra d20 module)
I also run plenty of sessions with no pre-authored scenario or situation, using systems that make it easy to establish situation and opposition on the fly.