This is a short list of recommended websites. The links open in a new window.
iClassicalScores - by Symphonic Apps (on iOS). A great place to store and annotate digital sheet music.
IMSLP - the best place on the internet to find public domain sheet music. An outstanding selection of over 75,000 works.
Violin Lab - teaching, guiding, and inspiring violinists of all levels through online violin video tutorials, community support, and feedback from experts.
Flute Tutor - this is a free (non commercial) information site, based in Australia, and it is now considered to be the leading such site in the country.
Amazing Slow Downer® - for sound quality, I consider this the best software to slow down or speed up audio tracks. Up to +/- 15% tempo changes can be employed with virtually no sound degredation, and even beyond this results are very good. The desktop (PC and Mac) software is good value at $49 plus taxes, but the mobile app is a good alternative and cheaper still. You can download a trial version of the software for free.
Audacity® - a free (open-source) wave editor which enables you to edit the tracks on this website to some extent. You can add or reduce periods of silence, mute or add click-beats and splice together loops or repeats. You can also change the pitch and tempos of the tracks, though the results aren't as good as with professional software.
MP3 Accompanist - this website contains accompaniment tracks to many vocal works, including operatic arias and lieder.
Pianotracks for Musicals - this website offers a fine selection of backing tracks for numbers from famous musicals.