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.
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.
an excellent tool for musicians looking to switch from printed paper to electronic sheet music formats. This company has a product line to cover just about all of your needs.
Paid, but with free trial and lite versions of software.