Magic Lantern is a firmware add-on, originally written for the Canon EOS 5D Mark II by Trammell Hudson in 2009, and ported to the 550D/T2i/Kiss X4 (1.0.8) in July 2010 by the same author. The current principal developer is known as A1ex and there are now versions for most Canon DSLRs.
Because installing Magic Lantern does not replace the stock Canon firmware or modify the ROM but rather runs alongside it, it is both easy to remove and carries little risk. The DSLR checks a “boot flag” in its re-writable memory, and if set, reads from a flash card to get the additional firmware routines. Each time the camera is started, there is an option to disable Magic Lantern. – Wikipedia
ML is being developed by photo and video enthusiasts, adding functionality such as: HDR images and video, timelapse, motion detection, focus assist tools, manual audio controls much more.