The Choral Public Domain Library (CPDL), specializes in choral music and has approximately 22861 choral and vocal works by at least 2713 composers. Apart from scores, you can find lyrics, translations, and information about composers.
CPDL supports advanced search so that you can search scores by title, composer, period, genre, voicing, and even lyrics. You can also browse in different ways such as alphabetically, by category, by composer, etc. CPDL is a community driven website, with over 500 contributors, so you can add your own scores and contribute to it.