Free Piano Sheet Music

Free web sheet music: A primer Free sheet music of amazing quality and variety is abundant on the web, but getting it can be can be frustrating. Manuscripts are in a variety of formats, often requiring "plug in's" to dislay and print the music; and there is no telling what you'll find before you get there. But a little patience and persistence can put you in touch with some gems, especially if you are in a browsing state of mind.

Index Sites: These sites don't have music; they list free music sites. Check them regularly for updates.

Classical Content: Classical music is easy to find (so much is in the public domain). These are the best of this class.

Sheet Music Archive A number of classical public domain scans, many with fingerings. Good first place to look.
The Chopin Files Most of what he wrote in good quality print, some fingerings, midi support, interesting background information. An A+ site.
Music Scores.com Good selection of composers, several difficulty levels, even some arrangements (e.g. flute & piano), full midi support.
Werner Icking Music Archive (formerly GMD Music Archive) Not primarily piano, but a variety of serious classical manuscripts for keyboard, strings, vocal; some midi support.
Daly Music = My Sheet Music Numerous classics (other tunes, too); for purists, authenticity of classic scores might be a question mark.

Popular: For free popular music, enjoy what you can get; it is unlikely you will find a particular number or arrangement you might have in mind.

OddRealm More pop tunes; when you click a number your are taken to the site where the number was originally found. Numerous dead links, though.
Music of Yesterday Straight forward voice with piano accompaniment of 200+ popular classics (in Noteworthy Composer format to look and listen).
PDMusic If your definition of popular extends to what might have been sung at home just after WWI, check these scans of "popular" songs.

Romantic, EZ Listen, New Age:

Almanac Creek Music Lots of mood pieces in excellent print out; no midi or mp3 makes reviewing dificult
Lori Line (pianist and composer) has a bi-monthly sample.
Brian Crain has montly samples.
Linda Kerby has a number of samples.

Folk & Christmas:

On Line Christmas Book Hymnal arrangements of most church carols
Daly Music = My Sheet Music Variety of easyish arrangements of carols

Other fine sites, especially at the price, but not my first tier:

Sheet Music Online A little awkward to use, but some goodies for piano and strings tucked away.
Mel Bay has a monthly freebie for piano.
Virtual Sheet Music Has a few free samples (classical mostly); great quality print
Mutopia Small but growing collection of short classics with midi support.
Masters of Classical Music Limited selection, but some lesser know items from Bach's Notebook for Anna Magdalena
Any Score.com Small collection of samples, most can be found elswhere
Musica Viva Grainy print with poor layout; good early keyboard focus.
Sheet Music USA Grainy print with poor layout; variety plus midis.
8Notes.com A lot of guitar mixed in, and demonstrates riffs more than complete pieces.
Composer.com Most content duplicated elsewhere; worth a look.
Piano Public Domain Might find these classics elsewhere; maybe not.
Nissimo Some teaching pieces plus you can submit your own work.