International Music Score Library Project
- a huge collection of scanned in sheet music that is no longer under copyright. This site is the impressive result of a worldwide effort, and probably the first place to look if you're searching for something.
Werner Icking Music Archive
- a site that features a wealth of sheet music, often in various interesting arrangements by artists in their respective field.
String Quartet Sheet Music
String Quartet Albums by Matthew Hindson
- two albums of popular string quartet music, excellently crafted and generously shared by Australian composer Matthew Hindson.
Mona Lisa Sound
- for string quartet, pop tunes (i.e. Beatles, Nirvana.) Because they are not in public domain, I am not—despite frequent requests—able to offer these selections here on my site, but Mona Lisa Sound does such a great job of it that it is hardly a loss.
- A terrific resource: 100+ pop/rock/movie arrangements for string quartet. Fully copyrighted and legal for purchase, these are also flexible, designed to work for small or full ensembles.
Vitamin String Quartet
- Also excellent arrangements of pop music.