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.
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).