For anyone who is at all interested in photography, the photographic collection of the Library of Congress is a treasure trove.
It's my belief that anyone who wants to call themselves a photographer should have at least a passing knowledge of the history of photography.
The Library of Congress collection provides a way to have a bit of fun exploring some of the history (mainly, but not only N. American) - from the early days of the medium itself, right through to close to the present day. Finding the work of not just some of the well known names from photography's past - Dorothea Lange, Roger Fenton, Matthew Brady, Fox Talbot, Atget, Walker Evans, Steichen, Stieglitz, Ansel Adams, Muybridge, Carlton Watkins and many many more, but also lots of "vernacular" photography as well - photographs of the gold rush town of Deadwood or panoramics of bathing beauties.
Not only that, but a good chunk of the collection is digitised - and in many cases the scans and files available are quite high resolution. Not every digital file has a hi-res version, but many do. This means that you can often download a favourite or interesting photograph, work on it in photoshop and print up a very presentable 11x14, say, to put on your wall (I have a number of such Walker Evans prints around the house) - oh and you can also order good old fashioned prints too.
In addition to the main online catalogue, there are also a number of selected collection in the American Memory site - thousands of photographs from the Farm Security Administration photographers (including a number in colour), Ansel Adams at the Manzanar Interment Camp, Civil War Images, Civil War - Brady Studio, the jazz photographs of WIlliam Gottlieb or Panoramic Photographs - to list just a few
(Lewis Payne - one of Lincon's assassins)
There also the amazing catalogue of Prokudin-Gorskii's tri-colour process images from turn of the 19th/20th Century Russia - the photographer to the Tsar.
(The Emir of Bukhara by Prokudin-Gorskii)
Now the interface in places is mildly clunky - mainly because they have been working on the digitisation and access programme for a probably 15 years at least and much of this was done before some of the main digital asset management systems came into being - but they are all workable (for example, when searching the main print and photograph catalogue, always be sure to click on "Preview Images" or you just get a text list).
(Stieglitz by Gertrube Kasebier)My experience has been that I go there to look for one thing and end up browsing around and finding all sorts of other stuff as well.