I'm happy to say that since my last visit to Pennsylvania's digital archives, their holdings have expanded nicely.
Their Online Research page takes you right to the source of several sets of data:
There are also numerous finding aids and research guides.
Two items of particular interest are ARIAS and Ancestry.com PA.
ARIAS offers up about 1.5 million historical records. The records are all military files from various conflicts dating back to the Revolutionary War:
This is a decent collection, especially for historians, genealogists, and family history researchers looking for records of their family's military roots.
Ancestry.com PA is just what is sounds like...access to the portion of Ancestry.com dealing with Pennsylvania. Access is free for Pennsylvanians when you enter a PA zip code.
Elsewhere, the Power Library, a joint effort by state government and non-profit organizations, has additional digitized resources, including: