Lifehacker just posted a review of Footnote, a great new site for finding and discussing original source documents on the web. Through a partnership with the National Archive, they have been uploading much material previously not available. But best of all is the slick 2.0 interface, rarely seen when seeking historical documents. Although there are paid subscription plans, access is free and non-paying members can even upload documents.
Also, on the topic of source documents, be sure to check out Lifehacker's post from last week on some of the best sites to find these materials.