American Girl has finally given their archived girls a permanent home on their website. It is called the American Girl Archives and while it is not prominently placed on their website, you can get to it from some of the inner pages. Here is the direct link to the American Girl Archives page – bookmark it.
The American Girl Archives pages only show the dolls that have been archived and a little paragraph about each one. If you are looking for a more extensive archive, I recommend:
AGPlaythings – the most comprehensive resource for retired items by American Girl from the historicals, to the GOTY dolls, to retired lines like Hopscotch Hill, Girls of Many Lands and more.
Complete Guide to American Girl Collecting – until recently this site was pretty well maintained and contains archived information and products, however, Curt Danhauser is no longer keeping it up to date so new items like Marie-Grace and Cecile and McKenna are not included. It is still a great resource though.