My favourite smell in the world is the smell of my ex-cat. You've probably heard of Dexter, but you've almost certainly never had a whiff of his glorious bouquet: it's redolent of stupid cat, obviously, but I miss it. Now I've moved away from him I might get an Olly, a little box that puffs out odours from your PC's actions.
It attaches to the PC's USB and whafts out odours according events, and what smelly stuff you put in it. Someone tweets? Why not have it puff out a tang of tangerine? Facebook updates could spread the smell of old tyres. Uninvited Linkedin invites? Why those smell like dog sick to me. It has a little drawer for inserting various smelly things, so all it takes is a slice of lemon to tell you when you have mail.
To make it even more futuristic, you be able to download it, or at least the 3D model files and parts, here. It's open source, so there's bound to be someone out there that can make it puff out the sweet smell of success (my girlfriend's armpits) whenever I stab someone in Team Fortress 2, right?