Printed music library online
If you are ever in an urgent need to take a glance on some classical piece – I highly recommend you this online printed music library. It’s in Russian (use Google Translate), but it has tons of scanned music, like, for example, all the most popular pieces for piano. As well as full orchestra score, jazz songs, choir etc. Maybe if you play a piece it will be better to get proper score (printed stuff doesn’t look so good and can be hard to read), but as a “take a quick look at that Chopin prelude” resource it is simply irreplaceable!