IMDb.com
   Contains a short biography, and a filmography of movies and television appearances.
   http://us.imdb.com/name/nm0000588/

Infoplease.com
   Contains a short biography.
   http://www.infoplease.com/ipea/A0762212.html

Rio's Attic: Friends of a Phoenix
   Biography and information about her relationship with River Phoenix.
   http://www.river-phoenix.org/friends/martha-plimpton/