IMSLP (the International Music Score Library Project) is the home of the Petrucci Music Library on the web. Their mission – to provide high quality music scores for free to those with internet access – has run into a few snags with copyright issues in some countries, but seems to be over that hump and running smoothly now. As of this post, there are over 75,000 scores available as downloadable PDF files on their site, and they are all free downloads with no registration. They have mostly classical scores with a few folk, traditional and modern originals.
The library is in wiki format and visitors may participate in the growth of the site. A useful side benefit of the site is that each entry has links to much more information. For instance, browse in the composer category and once you find your composer there are links to a list of compositions, a detailed biography and recordings. Or, once you locate a specific score, you will find a download link for the PDF file, but you may also find downloads for specific instrument parts, audio files of the work, a list (with links) to commercial recordings and general information about the selection including style/genre, performance key and possibly discussion from previous visitors. I don’t know what more you could want. I frequently browse for works by instrumentation (chamber orchestra, for example), which are relatively easy for me to convert to MIDI scores for synthesizer performances.