Once as a requirement for Speech class in college, but I was never able to keep it going. It turns out all I really care about are the basic facts of my eventful life (in opposition to my everyday life). Since 2006 I have been keeping an Excel spreadsheet of important events; the what, when, where, and who I was with. If something else of personal significance happened in the world on the same day I put it under the "In the News..." cell. Thanks to keeping movie stubs going back to 1989, I also include every movie I have ever seen in theaters, the theater I saw them in, and who was with me. Movies are important to me.
One day I had a revelation; add these events to Google Earth. I now have a complete world map of every significant thing I have ever done (and quite a few insignificant events). I am in the process of adding all the family photos and videos to it as well. Next year I plan to buy a 27" flat-screen touchscreen PC for the living room and have this spinning, virtual globe available to recall all of my past events. How's that for biographical documentation!