I have something of a weakness for free gaming modules. Whenever I go onto RPGnow to buy something I tend to also come away with a bunch of free pdfs, which build up and build up until I have a great pile of them; then I download them and skim them all in a blur of caves and ruins and dungeons with goblins in them. Most of them aren't that great, but a few really stand out; so I thought I'd flag a few of them up to bring them to other people's attention.
All of these are either freely available or 'pay what you want', at least at time of writing. Get them while you can, because PWYW products have a habit of suddenly acquiring fixed price tags after they get popular...
Dungeon of Signs: Gus L has produced a whole series of excellent adventures in pdf format, freely available on his blog. They're all well worth reading, but 'Prison of the Hated Pretender', 'Along the Road of Tombs', and 'The Dread Machine', are outstanding: dark and atmospheric and powerfully imagined throughout. Just steel yourself for a lot of dead PCs!
The Fungus Forest: It's not going to blow anyone's mind with its sheer originality, but this adventure module is a very solid example of OSR design principles in action: a subterranean forest full of weird factions and weirder individuals, almost all of which can be negotiated or allied with. Could make for several sessions of very enjoyable play.
The Caves of Moreau Country: This dungeon has a random-layout-generation gimmick, but that's honestly the least important thing about it. It's dark and vivid and sad and has some rather nice black and white art. It's also surprisingly faithful to the vision of The Island of Doctor Moreau itself.
Better than Any Man: I still think this is Raggi's best work, and he's giving it away for free. It's Lamentations, so expect cannibalism and dead babies, but if you're not put off by the subject matter then this is a very good example of historical fantasy-horror on a truly epic scale.
The Ruined Hamlet: This is mostly a pastiche of B2 The Keep on the Borderlands, and it's much more 'traditional D&D fantasy' than most of the others I've listed here, but it's done with heart and a pleasing level of attention to detail. Worth a look.
The Mysteries of Hollowfield: This is a series of short adventures set in and around a little woodland town, each of which is written by a different author. Unsurprisingly they're a bit of a mixed bag, but they mostly maintain a suitably creepy, Halloweeny tone, and the haunted house mini-adventure is a great example of using a few simple ideas and images to go a long way.
Madness of the Rat King: A 5th edition adventure, but should be very easy to convert to OSR D&D. This takes a concept which is cliched to the point of absurdity - a level 1 party gets hired to clear out some giant rats from under a tavern - and runs with it to such crazy lengths that it wraps right round into awesome again. By the time the flying rats with eye-beams attack, the cliches have been left a long way behind!