There have been crude attempts at this before, but this device — from, of all people, Scent Sciences — reckons it really can have you smelling napalm in the morning (provided the game you're playing has napalm and mornings).
The ScentScape is a small device that uses "hardware, software & algorithms and chemistry" to mix its cartridges of around 20 unique "flavours" into a distinct smell.
It says it can work for both PCs and consoles, users simply plugging in the unit via USB cable to be on the receiving end of "background scents such as pine forest or ocean" or "more scene related scents such as flowers and smoke".
Most games are obviously lacking the programming to release smells on their own, so it includes an editor that you can program so that you can at least try and synthesise the smell of the blood on your hands.
It's "coming soon", though an almost identical model already exists for movies and costs $70.