I had noted earlier that the Münchener Digitalisierungszentrum was hosting what appeared to be the complete works of Handel. They host a number of other interesting works as well, including the complete works of Mendelssohn, and have been adding new resources on a regular basis. Recent additions have included many volumes from the complete works of Robert Schumann.
One very helpful aspect of their site setup is an RSS feed, which makes it easy to see what new things have been added to the collection recently.
This digital collection’s offerings must be accessed one page at a time, which can make printing a long work slightly tedious. Also, one should sure to access the higher resolution scans with the buttons in the upper left hand corner. (For easier printing, many of these scans have been repackaged as PDF files at IMSLP.)
Finally, do note that some of the works in this collection may be public domain under European law, but are still subject to copyright restrictions in the US. (Particularly newer editions of ancient works.)