Mad God's Key had a really interesting feel about it - very much in the realms of a occult group being up to no good. Excellent low level adventure, too - one of the best I've ever run.
Reggie.
Shut in: a nice little adventure, where you can inject some good horror elements into it, and the PCs will get as much out of it as they put in.
Also second the recommendation of Shut-in (it's in Dungeon 128). My group really hated the big bad behind it and the psychopathic killer. (They were SO relieved to see him hung after the adventure was over.) They were very protective of the people they were hired to defend, and found the module pretty disturbing.
Bogged Down (Dungeon 91) is another good one. I ran it as part of the Kobold King series of modules and connected it with the Pallid Plague pathfinder scenario. Although I expanded upon it a bit, there is a lot of material there to work with. They found the swamp witch eerie yet pitiable. And the mastermind behind it all became so hated that the ranger PC planned an assassination job against him (failing) and the bard made a ballad of his ultimate defeat.
The Chimes at Midnight series* (Chimes at Midnight Dungeon 138, Quoth the Raven Dungeon 150, Hell's Heart dungeon 151 [[was online on Wotc's DDI website) has also ran pretty well. The first two adventures were disturbing and harrowing on several levels (particularly the second, that one allows for a lot of low blows and customization). There was some great roleplay moments and NPC/PC interaction (including the bard trying to reform a couple of the minor villains), and several literally OH S#$3 moments on the part of the PCs as discovered some dread revelations. I'm just starting the third part, and the run-up allows for some foreboding dread and foreshadowing, and from the first encounter they were disturbed by some of the things they discovered on one of the villains they defeated.
Fallen Angel* (Dungeon 117) allows for an exploration of a dystopic city in ruins. It's a bit different from the others in that it's less traditional horror or intrigue and more wondering about and exploring a mysterious fallen city and dealing with the degenerate cult that lives within. I liked this module because it put in enough details to fill in by the GM to lay the groundwork for many later adventures.
Escape from Meennlock Prison (Dungeon 146) is perhaps the most overtly maddening of the bunch I've run so far. The creatures inside this "prison crawl" were very harrowing to the party. They were very glad to escape, and I think they discussed razing the whole prison to the ground at one point.
*These modules were Eberron modules, but they fit in my homebrew horrow/steampunk world just fine with a little alteration.
Edit: Whereas any of the adventures might have some interesting things for Occult Adventures characters, Fallen Angel would potentially work the best for Occult Adventures, as it has plenty of places to customize the adventure (what are the ultimate motives of your employer, for example) or to stick stuff in. There's some eerie bits involving spirits at the end. The bard at the end might be retrofitted into one of the Occult Adventure classes.
Bogged Down might be another good one, as the swamp witch might be retrofitted into a shaman or some such. (She's merely an adept in the adventure, but she gives off this eerie vibe of something more.) Escape from Meenlock Prison and Hell's Heart might ostensibly allow for communication/haunting from tormented spirits if you decided to put some in.
In terms of general tone of what I imagine an Occult Adventure campaign (eeriness; Victorian tone/sentiments; the supernatural as wondrous, mysterious or dangerous), all of the adventures I've mentioned in this post seem to fit.
The Seeds of Sehan (parts I, II, & III) Campaign Arc
Vile Addiction: Dungeon 146 "A horrific drug has seized the population of the strange city of Exag, yet confronting its source only reveals the true extent of a dire new threat. For 8th level characters."
Spawn of Sehan: Dungeon 147 "Within an ancient necropolis a pool of alien slime grasps the minds and bodies of the people of Exag with wispy tendrils of madness. Those transformed by its power invariably seek the crypt, but what fell power draws them there? For 9th level characters."
Dread Pagoda of the Inscrutable Ones: Dungeon 148 "Atop a distant mountain peak, a monstrous entity plots dark deeds. For 10th level characters."