Contains collections on World War I, the Depression, Women's Lib and Honest Abe, among others. I'd imagine it will grow with time.
I've actually found Flickr much better as a source of quality historical images than Google Images or Wikimedia Commons.
I thought I'd added these already to the group, however the Diigo toolbar tells me I hadn't and he's never to be doubted. Please don't ask how I know the toolbar is male.
These collections are well-organised and high-quality. Focus on the US (for obvious reasons).