From the Magic Lantern site

Magic Lantern is an open platform for developing enhancements to the amazing Canon 5D Mark II full frame digital SLR. This camera is a “game changer” for independent film makers:

• It allows the use of a wide range of lenses (anything that can be adapted to the EF mount).
• The 35mm full-frame sensor is larger than the RED ONE’s sensor, Super 35 film. It is approximately the size of VistaVision. This means shallower native depth-of-field than anything on the market, except for the Phantom 65.
• The dynamic range and latitude are close to the capabilities of high-end HD cameras.
• The low-light performance is currently unrivaled, even by the RED ONE.

But, the software in video mode has limitations, even after the recent 1.1.0 upgrade from Canon that fixed the most glaring manual exposure “bug”.

That’s where Magic Lantern comes in — it turns your 5D Mark II into a 5D Mark Free. We’ve written extensions and widgets that fix many of the annoyances in working with the 5D Mark II on a film or video set. Our first set of fixes are targeted at the audio limitations of the camera, but there are some video enhancements included, too:

• On-screen audio meters
• Disabled AGC
• Manual gain control
• Zebra stripes (video peaking)
• Crop marks for 16:9, 2.35:1 and 4:3

Magic Lantern firmware introduction from Trammell Hudson on Vimeo.