The folks at Magic Lantern are no stranger to adding new features to Canon DSLRs, courtesy of their EOS Camera Tool software. Their latest creation - called Dual ISO - dramatically increases the dynamic range of the 5D Mark III and EOS 7D by four stops, bringing the total dynamic range to 14EV. This allows you to pull detail out of the shadows with a lot less noise than with the 'stock' firmware. The technical details are complex, but simply put, it works by interlacing two rows of pixels captured at ISO 100 with two rows taken at a higher sensitivity (usually ISO 1600). The low sensitivity rows capture highlight detail, while the high ISO rows capture shadow detail. When they are combined, you get relatively noise-free shadows without blowing highlights.

According to Magic Lantern, there are some downsides to using the Dual ISO firmware, though. Vertical resolution is reduced by half and there's more moiré and aliasing in over and underexposed areas. The author also warns that since this software modifies the sensor's operation, you could end up frying your camera. If you're feeling brave, you can find details on how to install the software here.

Dual ISO works for both stills and movies on the EOS 5D Mark III and for stills only on the EOS 7D. 

This example shows good shadow detail in the foreground, while the highlights in the windows are not clipped. Unfortunately, the author doesn't provide a 'before' image, so it's hard to see exactly what's improved.

If the example above was filmed at ISO 100 to preserve highlights (in other words, underexposed), the foreground would be dark. If you tried to bring up the shadows in post-processing, noise levels would be very high. Filming at ISO 1600, on the other hand, brightens the foreground but clips the highlights in the windows. By combining these two images, you get the best of both worlds.