Harp Music Wiki - http://music.lib.byu.edu/harpwiki/index.php/Main_Page
Resources for harp music of the 18th & 19th centuries, including biographies of composers and downloadable facsimiles of manuscripts and rare editions.
Reigning Harps http://www.reigningharps.com/ a harp club in Seattle, offering new arrangements or original free sheet music for Celtic harp posted quarterly. The older posts are also archived, and include some arrangements for harp ensemble.
8 notes.com http://www.8notes.com/ - Free Sheet Music, Riffs, Lessons and Tools for musicians
http://imslp.org/wiki/Main_Page - IMSLP / Petrucci Music Library, a public domain sheet music library with thousands of free downloadable solo, chamber and orchestral scores and parts.
A Harp in the School: A Guide for School Ensemble Directors and Harpists (2006), edited by Chelcy Bowles, available at www.astaweb.com in their publications section.
Education Project 2000 - a listing of published harp parts for school ensembles (grades 1-12); Harp in orchestra; Harp in band.
A personal collection of Edna Phillips Rosenbaum, principal harpist of The Philadelphia Orchestra (1930-46), harpist for the Fantasia soundtrack, and the first woman to hold a principal position with a major American symphony orchestra. She and her husband Samuel Rosenbaum commissioned some of the most significant works in the harp repertoire, such as the Ginastera Harp Concerto. Acquired in 2004 by the University of Illinois at Urbana-Champaign, the Collection was donated to Illinois by her daughter Joan Solaun.