IMSLP: Petrucci Music Library
This is my favorite website for finding open-source classical music since it has a strong community who, among other things, rates the quality of the sheet music. There are also recordings of some songs, which is massively helpful.
CPDL.org: Free choral sheet music
Here you’ll find sheet music for songs with lyrics, like anthems and children’s tunes.
I really love this website. You have the option to listen to and download recordings and sheet music, and it has a shuffle button if you want to randomly discover songs in their library.
This is the portal for the Royal Conservatory of Music, the Canadian standard for qualifying skill. When people say “I have my grade 9”, they’re referring to this. They offer exams and books and the focus is classical music, with some modern. I always recommend these to my students, since following a well-tested, structured learning path and taking exams is really useful in the pursuit of excellence.
Royal Conservatory Music Development Program
This is RCM (from above), but in the USA. It’s a relatively new program, and still expanding.
ABRSM.org (Associated Board of the Royal Schools of Music)
This is the British version of Canada’s RCM, also popular in Australia/New Zealand.
A program that enables you to create your own sheet music for free.
Now you don’t have the excuse of practicing without a metronome. Also has an app for those who don’t have a computer near their keyboard.