I have been watching the resources list that has been created by the University of Pennsylvania Libraries with a lot of interest over the last few months.
Whoever is building and updating this list of links is doing FANTASTIC work in my view. Most link sites just provide the link, but the team at the UPenn Libraries is adding some very useful information to each link which assists you in deciding whether to explore the link.
The list is rapidly growing and since I subscribe to the updates, it is a regular "read" for me.
I highly encourage you to visit often the Historical Newspapers Online Guide that the library makes available.
It is excellent and they do a great job!