AreusTech recently revealed a Wii peripheral entitled "Smells Like Wii Spirit", a device that produces different odors based on what Wii game you are playing.
Connected to the Wii via the system's USB port, the device will respond to whatever Wii game is currently being played. Examples cited by AreusTech include Mario Kart Wii (which will produce a burning rubber smell) and Tiger Woods PGA Tour 07 (a freshly-cut meadow). AreusTech has promised that the device is compatible with all curent Wii titles, and will be compatible via updates with new games.
Smells like Wii Spirit will be available in Germany for the price of 149 Euros and will be released in July of next year, with no known plans of release elsewhere.