Here you'll find links to a number of online sheet music collections that allow you to download and print complete works. These are generally in the public domain (copyright-free), but be sure to check the usage guidelines on each site beforehand!
Directory of large sheet music collections accessible either through a web page or an online catalog. Many of the collections include digitized reproductions of sheet music in the public domain. Hosted by the Duke University Library.
The VARIATIONS project is a prototype project, created by Indiana University, that combines musical scores, recordings, and liner notes, for a number of works (opera, orchestral, choral, solo instrument, etc.).