(LOC) has published a finding resource listing 71 links to the 50 states, online digital collections. That is found here .
The PA State Library — Has a digital collections, very similar to the digital collections found at seekingmichigan.org [Editor: also in LOC list for MI].
From Abe Lincoln, to Ben Franklin, to Coal Mining History, to WWI there are many PA treasures here:
I chose to start in their WWI Collection, which had a few choices to pick from, so I chose the top pick (Mahanoy City):
American Red Cross. Pennsylvania Chapter. Mahanoy City. In Memoriam Of Those Who, Coming from the District within the Limits of the Mahanoy City, Red Cross Chapter, Quakake to Girardville [inclusive] Made the Supreme Sacrifice in the Great War for Democracy, known as “The World War” 1917-1919. Mahanoy City, Pa., 
This is a six page memorial to the fallen veterans who lived in Mahanoy City in the anthracite coal region of northeastern Pennsylvania.
In truth the PA State Library’s digital collection is large enough that this jester will need to spend some time exploring, but I thought I would share my initial impression.
So LOC, a tip of the jester’s hat to you for compiling a very useful resource of state libraries who have online digital collections. These are historical in nature, but the obvious application to genealogy make these valuable resources to the genealogical researcher too.