There are several notable online digital image collections that have great value to genealogists and rarely get noticed or mentioned in the lists of valuable sites. Most of these sites are available through the respective state archive sites where the collections are located. Perhaps their lack of exposure is a result of the state's restricted budgets, but I suspect it is the case that the commercial genealogy sites get a lot more attention due to their constant advertising. All of these sites (and many more) provide free access to the records.
First on my list of great sites is that of the State of Georgia. Their main collection of online images is included in their Virtual Vault. Particularly helpful are the Colonial Wills, County Maps, Georgia Death Certificates, Historic Maps, Militia Enrollment Lists, 1864, Spanish-American War Service Summary Cards, Headright and Bounty Plats and many, many more.
Also notable is the Virginia collection at Virginia Memory. For example, the Chancery Records Index has over 175,000 cases indexed with a total of 2,375,233 images of chancery causes available online. The Chancery Records Index (CRI) is a result of archival processing and indexing projects overseen by the Library of Virginia (LVA) and funded, in part, by the Virginia Circuit Court Records Preservation Program (CCRP). Each of Virginia's circuit courts created chancery records that contain considerable historical and genealogical information. Because the records rely so heavily on testimony from witnesses, they offer a unique glimpse into the lives of Virginians from the early 18th century through the First World War. I find the images a little difficult to view and, in some cases, they need to be downloaded and then viewed.
The State of Washington has 65,429,432 searchable images online. These records include King County Marriage Records from 1855 to 2000. The marriage records collection alone contains over a million records.
The Wyoming online map collection is also notable. The Historic Map Collection has more than 12,000 over-sized maps, plans, certificates, posters, and newspapers.
There are so many resources online that it could be a full-time job just keeping up with the collections.