I came across IMSLP, also known as the Petrucci Music Library, in November 2007 from a BBC article:
In February 2006, a part-time Canadian music student established a modest, non-commercial website that used collaborative wiki tools, such as those used by Wikipedia, to create an online library of public domain musical scores.
Within a matter of months, the International Music Score Library Project (IMSLP) featured more than 1,000 musical scores for which the copyright had expired in Canada.
Within two years – without any funding, sponsorship or promotion – the site had become the largest public domain music score library on the internet… Source
Lawyers acting for Universal Edition, the Austrian music publisher, then hit IMSLP with 'cease and desist' notices, demanding that Austrian copyright law be applied to scores held on a Canadian server.
A tremendous brouhaha ensued, with music student (David) up against Universal Edition (Goliath) and, effectively, the future of e-commerce at stake:
According to Universal Edition, businesses must comply both with their local laws and with the requirements of any other jurisdiction where their site is accessible – in other words, the laws of virtually every country on earth.
It is safe to say that e-commerce would grind to a halt under that standard since few organisations can realistically comply with hundreds of foreign laws. Source
David won and IMSLP now hosts over 69,000 scores representing over 2,500 composers, making it the largest public domain music score collection on the web. It's run collaboratively by volunteers organised on a forum which anyone can join.
In 2009, IMSLP won the MERLOT Classics award for Music and was named one of the Top 100 Web Sites of 2009 by PC Magazine. It gets millions of hits a day.
IMSLP is a brilliant, high-minded project which believes "access to our culture and the Arts is a fundamental right of every human being." Amen. Not surprisingly, that rubs up some music publishers the wrong way.