LBI has been placing parts of their holdings online at archives.org since 2006. I've seen fascinating stuff such as unpublished correspondence of Gershom Scholem and Cecil Roth. I just recently noticed that they have placed the entire Joseph Perles collection online.
There are many, many fascinating letters from such well known figures as R' N N Rabinowitz, the Dikdukei Soferim, Shadal, S. Buber, as well as from many lesser known but equally important personalities. Knowledge of German and Italian (which I don't have) is necessary for most of the letters but there is also a nice percentage of letters in Hebrew.
I hope to make use of some of this material on this blog over the next few weeks. If anyone finds anything interesting there that they would like to share, I'd appreciate if they can leave a comment or send me an email.