For years, IMSLP has been my go-to for free piano music in the public domain. But when I was compiling my Rote Repertoire for Piano notebook recently, I came across another source loaded with free piano music:
Michael Kravchuk is a pianist, violinist, teacher, composer, music director, and book publisher currently living in Sacramento, California. In addition to free piano music, he also has typed and edited violin music, vocal music, lead sheets, trumpet music, and a few worksheets. His free piano music collection includes piano solos, arrangements, original compositions, piano/violin duets, piano/voice duets, sightreading, and beginner songs. Some of the categories are sparse, but he seems to be adding to the site regularly with new material. Definitely a resource worth bookmarking and utilizing often!