Come Home to Little House http://www.littlehousebooks.com The official Harper Collins website for the series. This is the best place for information on HarperCollins products and includes some background information, teaching resources, crafts, and trivia.
Beyond Little House http://www.beyondlittlehouse.com This is the official website of the Laura Ingalls Wilder Legacy and Research Association that hosts a Laura Ingalls Wilder conference every two years.
Bittersweet http://thelibrary.org/lochist/periodicals/bittersweet/wi79b.htm A publication that aspired to be a Foxfire for the Ozarks, they published many good articles. This is a transcript of their 1979 piece on Laura. It's outdated, but still a nice article to know about, especially for the interview with the Lichtys which is included.
Deb Houdek Rule's Laura Ingalls Wilder Page http://www.dahoudek.com/liw/default.aspx Another one that doesn't get a lot of updating, but has some basic information and a very good genealogy section.
Frontier Girl http://www.liwfrontiergirl.com/ Although she doesn't update the page anymore Frontier Girl has a lot of good basic information.
Laura Ingalls Wilder at the Hoover Library-Museum http://hoover.nara.gov/LIW/index.html The Wilder papers are housed at Hoover and this site not only gives research information, but also outstanding teacher information, including a Dear Laura to allow you to ask questions.
Pioneer Girl http://www.pioneergirl.com One of the most authoritative websites of Wilder information and research.
Prologue From the National Archives http://www.archives.gov/publications/prologue/2003/winter/little-town-in-nara-1.html An Archivist walks you through some of the Laura Ingalls Wilder records available through federal documents. Be sure to click on the link at the bottom for the second part. If you find a paper copy (the National Archives sells back issues) it's worth getting the paper version which has many, many more images of photographs and documents.
Purple House http://www.purplehousepress.com/liw.htm Some good information about the older editions, although I'm doubtful on their right to publish the images they used.