While apparently no cause of death has been specified, Margot Kidder's manager says she passed away peacefully in her sleep. (See for example http://www.abc.net.a...aged-69/9761342, or any internets.)
Obviously Kidder was the definitive Lois Lane now and forever, but I will also remember her fondly for her time as Gaia on Captain Planet, her visits to Smallville, and her narration of The Wonderful Wizard of Oz.
(Is it possible we don't have a thread for this yet? Did I fail to find it? Surely.)