The International Music Score Library Project (IMSLP) was launched five years ago with the stated goal of "creating a virtual library containing all public domain music scores."
The website, also known as the Petrucci Music Library, has made enormous strides towards that goal, now containing over 85,000 scores in PDF formatone of the largest collections of public domain sheet music anywhere, let alone on the web.
The IMSLP operates on the "wiki" principle known as crowdsourcing, allowing users to upload scores, edit metadata, and moderate the site.
Although abuse of the wiki format got the site into serious trouble with music publisher Universal Editions in 2007, it has maintained stricter copyright enforcement since its relaunch in 2008, and has still been growing at an exponential rate.
The unprecedented free library has become enormously popular with music students and teachers.
A large percentage of the classical repertoire was published before 1923, meaning that is in the public domain in the US and can be legally downloaded and printed from IMSLP.
A trombone student needing a part for Mahler's Third Symphony to prepare for an audition no longer needs to rent or buy a hard copy. A music theory professor can quickly throw a PDF score of Mozart's Clarinet Concerto on the projector and point to musical examples, without the need to scan in each page of the music.
As far as it has come since its launch in 2006, IMSLP has even bigger goals for the future. The site recently began hosting recordings of ensembles willing to freely share performances of public domain work.
Its Goals page also states that it hopes to "facilitate the exchange of musical ideas outside of compositions: for example, the analysis of a particular piece of music."
Like Wikipedia, IMSLP is only limited by the creativity of its users, and there is little doubt that it will continue to revolutionize the sheet music industry.