Song of the Starwalkers
Here's a scenario I wrote and ran back in '07 for Call of Cthulhu 5th Edition. The campaign was aborted, but I think this investigation holds up.
Red Eye of Azathoth
I really dig this sprawling historical campaign centering around the primal Mythos deity of chaos.
I ended up running the first two scenarios using Cthulhu Dark, but if you're looking for a way to plug some horror into Risus: The Anything RPG, here are some ideas (pre-generated PCs and a sanity mechanic).