I have really enjoyed looking through the Library of Congress photosets on Flickr.
From The Commons description:
The key goals of this pilot project are to firstly give you a taste of the hidden treasures in the huge Library of Congress collection, and secondly to show how your input of a tag or two can make the collection even richer.
There are two sets of photographs: News in the 1910s and 1930s-40s in Color.
Over the course of two or three weeks, I managed to see all 1500 photos in the News in the 1910s photoset. If you are a baseball fan, you will find photos of Cy Young, Germany Schaefer, Russ Ford, James Mullen, John Titus, and many, many more. If boxing is your sport, there are pictures of Jack Johnson, Kid Sullivan, and Mrs. Edwards and Fraulein Kussin to name a few. Interested in politics? How about some pictures from the 1912 Democratic and Republican National Conventions? There are even pictures of the Titanic survivors reaching New York.
There are 1,615 pictures in the 1930s-40s in Color set. I have barely made my way into those, but I am looking forward to finding a lot of great treasures there, too.
By the way, the Library of Congress also has a blog.