Burning plastic.
If you've ever had a plastic spatula or something fall onto the heating element in your dishwasher, and then been assaulted by the overwhelming fumes when you opened the dishwasher... then you know why I put it first on my list. It's far, far worse than the most pungent, putrid poop. It's also toxic: it can contain dioxin and carbon monoxide, among other less-than-desirables. And the choking fumes seem to soak into every porous and semi-porous material in the house and just stick around, no matter how long you freeze yourself with your doors and windows open.
Some other nauseating smells, in no particular order:
Rotting garbage
Formaldehyde (thanks a lot, biology instructor!)
Tobacco smoke
Beer
Outhouses
Baby diapers
Baking parmesan cheese
Skunk spray
Cat pee
Pig manure
Cow manure
Most anything deposited in a steaming heap by a wild animal
Vinegar
The football team's locker room
Dad's farts
Burnt Ryvita rye crackers (thanks a lot, mom!)
Stinky diesel trucks
Exhaust from an oil-burning junker