Though they are more like setting book rather than adventure, I'd suggest The Deadful Secrets of Candlewick Manor. I GMed a few games for it, and the setting is simply amazing.
Another great alternative would be The Mysteries of Mesoamerica by Pagan Publishing. Though it has a few adventures, I could definitely see running an entire campaign by linking those adventures together.
Delta Green, as Ross said, can be easily converted into a long adventure. Eyes Only features many more adventures, as well as expanding on the Fate. And Targets of Opportunity could provide great starter enemies, ones that could be more easily defeated than the big players but that could link to the others in later scenarios.