Carl Nielsen EditionLinks take you to the electronic Carl Nielsen Edition (link is external), “a collected, critical edition of Nielsen's works in 32 volumes plus study scores and parts. Each volume has an introduction, describing the history of the work and a critical report containing a survey of the sources and a list of all emendations and variants.”
Carl Nielsen Works Catalogue (CNW).This is the “first ever thematic-bibliographic registration of all Nielsen’s compositions.” In total, 414 works are registered in the CNW, including stage music, instrumental music, and vocal music. Information is included about the history of the piece and early performances. You can search by keyword, limit to a time period, and filter by genre using the help of color-coded dots.
IMSLP - Petrucci Music LibraryCanadian Web site sponsored by the International Music Score Library Project and dedicated to creating "a virtual library containing all public domain music scores, as well as scores from composers who are willing to share their music with the world without charge."