Check out Tuna Knobs, real knobs that work on any ipad, tablet, touch screen, you name it, old school for the new millenium! Thanks to Samuel Verburg these things are real!

Line’em up with the virtual knobs on screen, a grippy acrylic base and suction cup holds them in place, and you have transformed a real-life turning movement into an in-app touch command
Sam hooked up with Tweetonig and figured out how to get the touch and feel of the old style knobs onto a tablet. There’s a prototype, there’s a Kickstarter campaign, Pledges vary from €9 (US$12) for a single knob up to €85 ($115) for a set of 10.

They are working on getting compatibility with the TouchOSC, Korg iMS-20, Korg iElectrive and iDJ2GO,.
Tuna Knobs are compatible with Windows, Android and iOS touchscreen devices.