Air guitar is something that doesn't seem to need much technological assistance. After all, it's just pretending to play guitar. But if you want your air guitar to be a little less air and a little more guitar, there's now an iPhone accessory for that.
The Air Guitar Move is a Kickstarter project for a guitar pick that you can plug into your iPhone. Then, using a custom app, your iPhone will play sounds based on your air strumming.
It's a neat idea, I guess, but I also think that one of the main benefits of air guitaring is that no one can hear what you're doing. If you add sound to it, you might as well just get a real guitar.