IMSLP stands for International Music Score Library Project. The logo [which appears at the top of the home page] is a capital letter A, taken from the very first printed book of music, the Harmonice Musices Odhecaton, published in 1501. Its author, Ottaviano Petrucci, is this library's namesake.
While working on an Examiner.com preview piece for a concert by the San Francisco Chamber Orchestra featuring Dmitry Sitkovetsky’s arrangement of Johann Sebastian Bach’s BWV 988 (better known as the “Goldberg Variations”), I was delighted to discover that this site included a PDF file of a facsimile of the first edition of this music in Bach’s series of Clavier-Übung (keyboard practice) publications. This is not the sort of thing you are going to prop up on your piano’s music stand, but it is about as close as you are going to get to Bach’s own account of this body of music. The Internet really is changing everything, at least when it comes to sparing those of us with limited resources from having to travel to distant libraries, where we then have to worry about handling anything from the stacks with extraordinary care.
Indeed, one could get lost just reading the “Featured” box on the IMSLP home page. If BWV 988 is not enough, the autograph manuscript of the BWV 582 C minor passacaglia has recently been added to the collection. For those with more nineteenth-century tastes, there are also autographs of Johannes Brahms Opus 90 F major symphony (his third), Camille Saint-Saëns’s Opus 22 G minor piano concerto (his second), and one of the great hits of bel canto opera, Gaetano Donizetti’s Emilia di Liverpool. (Say what?) The source pages for all entries are kind enough to provide both pages counts and file sizes, so you have a pretty good idea what you are getting into before clicking are a download.
The whole site is implemented as a wiki. So, from a look-and-feel point of view, it follows the basic conventions that any Wikipedia reader (or contributor) knows and loves. This also means that all contributions are the result of voluntary efforts, which implies the corollary that there are a lot of people out there (probably of varying levels of music scholarship) putting in generous amounts of time for the common good of those for whom “music” means something more than using iPod ear-buds to create “background insulation” from the harsh assaults of the “real world.” It also appears that these volunteers are less inclined to the “fight club” mentality that has undermined the value of Wikipedia in certain subject domains. It is nice to know that, where the sharing of scores is concerned, the community of those who both love and study music is a comfortably stodgy bunch!