A huge domain of choral music, fantastic for harmonic analysis/ familiarity with SATB scoring and class performance (some easy canons all the way up to complex anthems, secular & sacred).
Although similar in some ways to IMSLP, this website focuses on new free editions of choral music. This is a fantastic resource for anyone working with choirs or who needs editions of renaissance music that has been modernized and added translations or keyboard reductions.
This is an extensive resource for both choral and solo vocal music, for any compositions that are out of copyright. This is a free website and often contains pdfs, mp3 files and Finale or Sibelius files of the pieces. It tends to be focused on Western Classical music.
I really like this website because it's an excellent tool to use when trying to teach about different periods of music and their characteristics. Instead of digging through your music library, you can quickly scan imslp.org to see if a piece of music you'd like to analyze is available. Very awesome tool both as a student and a teacher.
This is also really useful for looking at original scores of early music, to explain the origins of notation and the development of the stave, barlines, etc.