TMDtouch of Shenzhen, China has released Zorro Macsk, an external multi-touch overlay for Apple's iMac line. Unlike similar hardware from TrollTouch, the Zorro Macsk supports gestures and multiple points of contact. Engadget went hands-on with it, and posted mostly positive impressions.
What's cool is that there's no software to install. To get started, simply affix it to the front of the iMac and connect it to a USB port (all the iMac's ports remain accessible). From there, wait about 15 seconds and begin. It even ships with a capacitive stylus, which I assume you can share with your iOS devices.
Performance was hit or miss. The Engaget folks found single-taps to be reliable, while double-tapping was not. As they said, this could be due to that fact that OS X wasn't designed to be a touch-friendly operating system. Gestures were just as finicky. Multi-touch scrolling worked well while pinch-to-zoom was stubborn.
It's worth considering if you're in an environment that could use such a device, like a school. The 21.5-inch model is priced at US$199 and requires no modifications to your iMac.