The manuscript of Mahler 5 is now available online here. Alex Ross mentions that the Morgan Library will put on-line digitized versions of hundreds of manuscripts. Already available are Mozart's Haffner Symphony and Schubert's Winterreise
Here's the opening of the famous Adagietto:
It's always amazing to look through a composer's manuscript. I wonder if we will have such experiences in the future, with many (most?) composers using music writing software. What will be lost, not seeing the individuality of handwriting, cross-outs, scribbles? Hard to know, but this is a beautiful manuscript!
I own several facsimiles, including Bach's Mass in B Minor and Mahler 7. It was interesting to answer a question a couple of the grad students at UNT had about a spot in the Bärenreiter edition (based on the Neue Bach Ausgabe) with the facsimile last year.