This source isn't a primary source site itself, but has links (22 at time of posting) to sites that help students analyse primary documents. Might be useful.
This is a guide to creating lessons using primary sources provided by the Library of Congress. They've just started a new initiative helping teachers use primary sources in the classroom; I went to the site they're set up but there wasn't much there. Maybe it will grow in time.
A site with primary sources that also guides students in source analysis/evaluation. It's run by George Mason University (they do a lot of the heavy lifting when it comes to open-access digital history. Good on 'em!).
Another site to help students use primary sources. Has some good questions to help students develop their eye for historical detail, motive, perspective, etc.
Has some good questions for students to ask themselves when evaluating primary sources. Such a difficult skill to train students to do - it seems to me though that historical knowledge is vital for students to be able to analyse and evaluate sources effectively.
This isn't strictly speaking a site with resources usable for historical teaching and research, however it has a great breakdown of why historians use primary sources and how we use them etc and this might be useful with your classes.
An excellent newsletter to sign up for. They're not just try to make money and they are historians and know what they're talking about. If you're on twitter then #historyteacher is the place to be!