Hebrewbooks.org has added some important new Seforim. This is one of the most important. I will try to update with some more info later.
They also have the original Leipzig edition of Zechariah Frankel's Darkhei HaMishna, although it is best to see the Warsaw edition as well as there are important additions there.
Also of interest is a response by Mordechai Weissman Chajes to Chaim Zelig Slominsky, containing important treatise re: the dateline. Mondshine's biography of Kamehler is also worth examining.
These are only some of the important Seforim added too Hebrewbooks.org. I will try to update this post significantly later today.