What is it?
Magic Lantern is an enhancement atop of Canon's firmware that frees your Canon DSLR, allowing you to use many useful features. Initially developed for filmmakers, it now has functionality for both photo and video enthusiasts, including manual audio, zebras, focus assist tools, bracketing, motion detection and much more. It is an open (GPL) framework for developing extensions to the official software. It does not replace the existing firmware, but instead runs along side of it. There is no need to "uninstall" it -- simply format your card to reboot to the stock Canon firmware.
Is it only for video?
Initially, Magic Lantern was developed by independent filmmakers and tailored for video production on 5D Mark II. Things changed when Magic Lantern was ported to smaller (APS-C) cameras, like 550D, 60D, 600D and 500D, which attracted developers interested in both still photography and DSLR video.
Where do I get it?
See the Download page.
Will it break my camera?
As of September 2009 the software has been downloaded over two thousand times and there have been no reports of damage to the cameras. While this is no guarantee of absolute safety, the stable releases have been tested by beta testers. Most of the risk is to the developers' cameras while testing new features and probing new portions of Canon's firmware. By the time the software moves from development to beta testing it has been installed hundreds or thousands of times.
What can I do to help?
If you're a programmer skilled in ARM assembly, embedded systems, GUI programming and don't mind risking your expensive camera, get the source code, join the forum, edit the wiki and make improvements.
Where do I report bugs?
Why not just buy a video camera?
If you can find a video camera that a) shoots HD, b) has a 50 mbps data rate, c) has interchangable lenses, d) has a 35 mm or larger sensor and e) costs less than $25k (without lenses, like the RED One), then buy that one instead. There are limitations to shooting movies on a 5D Mark II, notably the limited 12 minute recording time and lack of balanced audio inputs, but a ArriCam Lite only records 5 minutes of Super 35 and a high quality preamp like the juicedLink CX231 provides balanced inputs. The lack of auto-focus in movie mode isn't a problem either -- movies are focused manually with a follow-focus like the Cinevate Durus.
Will it work on my camera?
Check the nightly builds page - if your camera is listed there, it already works.
If your camera is not listed on the nightly builds page, the short answer is: maybe. Longer answer: The CHDK project successfully supports many different cameras running different operating systems, but finding the necessary kernel entry points is a very time consuming process. Since Magic Lantern is publicly available, someone with the time, the hardware and the inclination to port it to other cameras can do so.
For 400D, use 400plus.
For Panasonic GH1 and GH2, please check http://www.gh1-hack.info/. This is a completely different bit of hardware and would require an entire from-scratch reverse engineering effort in order to port Magic Lantern.
For Nikon cameras see http://nikonhacker.com/.
There is also an effort in reverse engineering the Pentax K10/GX10 and K20/GX20 cameras. As of August 2009 they have been able to decrypt the firmware update and are making progress in understanding how the camera works.