The European Library OnLine catalog for major European research libraries, many with extensive holdings of digitized sources. http://www.theeuropeanlibrary.org/tel4/
Early Modern Web: A Round-up of Digital Projects on Subjects before about 1800. http://earlymodernweb.org/
International Music Score Library Project (IMSLP) Public-domain scores, organized by composer. Many facsimiles here, some of them provided by European and North American research libraries. http://imslp.org/