I've been gathering material to run a Delta Green campaign using the Nemesis rules. The One Roll Engine rules look like they'll make for a pretty deadly game and the Madness Meters offer a different kind of insanity experience from BRP CoC's Sanity system.
I've found the Sanity system in CoC leads to a death spiral that rapidly leads to an unplayable character; not a great design for a campaign game. The Madness Meters used in Nemesis offer a gradual hardening of the psyche that leads to various forms of sociopathy, along with the gradual onset of various other mental disorders. See? Much more entertaining! It's a little more complicated than that, but it does seem to allow for a little more variety and playability in the gradual slide toward psychological entropy.
The combat system offers limited hit points, non-lethal and lethal damage, hit locations and a variety of light tactical options for players. It's interesting, but lethal.
Nemesis has a somewhat unfinished feel and it was never released as a for-profit publication, but it does have the essentials that you can use to build your own horror game.
So far I've gathered material from a series of posts converting various Cthulhu-mythos spells to Nemesis and I've begun assembling conspiracy material for cults and organizations working behind the scenes (much of it from The Fairfield Project and The Yellow Site).
There are a few good Delta Green scenarios floating around out there, Night Floors is definitely weird and creepy, you can use source material from Insylum to get the background for the Carcosa stuff.
You can find the spell conversions here.
Mythos creature conversions (WIP).
House rules for Nemesis/Delta Green (WIP).