Thanks to a tip from the AncestryInsider.org, I was reminded of a online list of Microfilm Publications and Original Records Digitized by Our Digitization Partners from the U.S. National Archives and Records Administration (NARA).
The list contains links to each of the digitized collections. Of course, you will have to have a subscription or other access to each of the three websites where the documents are located. This is explained by the screenshot above. Here is an easier to read copy:
Our digitization partners, Ancestry.com, Fold3.com, and Familysearch.org, have digitized selected NARA microfilm publications and original records and made them available on their web sites. Familysearch.org is a free site. Ancestry and Fold3 are both subscription services that allow free searches of some or all index terms for each title. Complete access to subscription sites Ancestry.com and Fold3.com is available free of charge in all NARA Research Rooms, including those in our regional archives and Presidential libraries.
The list below includes all microfilm publications and original records that have been either partially or wholly digitized by the partners. The list will be updated when additional materials are digitized.I am guessing that the comment about updating the list is based on some annual review because the date on the list is 17 December 2014, about six months ago. The original records are towards the bottom of this very long list and most of those records are on Ancestry.com.
This list may give you an idea of the numbers of records that are being made available through the large online genealogical database websites.