Remember Soundracer? (Makes your car sound like it has a different engine... gadget plugs into your lighter outlet to read your rpms, then sends appropriate audio to your stereo. I think they sell for around $40.)

Well, now there's a free alternative, an app for iPhone and Android. It uses the phone's accelerometer instead of plugging into lighter socket.

Hoaky, but a novelty I thought I'd share: Link to article with video and links to download.