Footnote is the place to view historical documents online. It is fee based ($60 a year) but one can explore the site before deciding to join. Its strength is in providing the actual images of documents. You can search indices of millions of documents for free but if you want to see or download the images there is a small fee.  NOTE: If you are at NARA or Family Search, it is free!)

  It is interactive and you can upload scans of your own documents. There are 200,000 registered users who also make regular contributions. This content is not prejudged but may be edited by other members. If you see a document of interest you can attach annotations to it. You can also have a personal gallery of your information and images. 

The scope of Footnote is huge! It now includes images of British censuses, military records, veteran pension records, immigration records, petitions for citizenship, declarations, naturalizations and some slave emancipation documents, images of vital records, (some of which include relationships that you cannot find elsewhere,) City Directories: Military and immigration records, especially Revolutionary War and Civil War records. Some mortality schedules (which are not easy to find.) The 1860 Federal Census is now online (and more are coming.) 

Beau showed a searchable image of the Viet Nam Wall Memorial and when searched it provides an enlargement of the name. Very impressive!

The site has partnered with NARA, Family Search, Allen County Public Library and the Pennsylvania Archives and many others.

(There is a 7-day free trial but the seven days start as soon as you sign up.) 

To get a free tour of Footnote go to http://www.footnote.com/tour.php