PBS
New Perspectives on the West: Marcus Whitman & Narcissa Whitman
PBS site detailing the Whitmans' journey to Oregon country, their establishment of a mission, and their death at the hands of the Indians. Also includes some background information.
Library of Congress
Loc: Today in History: February 16: Missionaries in the Oregon Territory
This Library of Congress site briefly describes the missionary movement to Oregon, centering around Marcus Whitman.